/^BERKELEY 

LIBRARY 

UNIVI 
V       CAL 


UNIVERSITY  OF 
CALIFORNIA 


CM  FT   OF 
Professor  Max   Radln 


GEOLOGY 


Appleton  s  Scientific   Primers 

Edited  by 
J.  Reynolds  Green,  Sc.D. 


BIOLOGY.  By  Prof.  HARVEY 
GIBSON. 

CHEMISTRY.   By  Prof.  W.  A. 

TlLDEN. 

BOTANY.  By  J.  REYNOLDS 
GREEN,  Sc.D.,  F.R.S. 

GEOLOGY.  By  Prof.  J.  W. 
GREGORY. 


Scientific      ^Primers 
Edited  by  J.  ^ynolds  Green,  Sc.T>.,F.R.S. 

GEOLOGY 


BY 


J.    W.    GREGORY,    F.R.S. 

Professor  of  Geology  in  the  University  of  Glasgow 


WITH  NUMEROUS  DIAGRAMS 
AND  ILLUSTRATIONS 


New  Tork 


(D.    *Appleton   and.  Company 

M582213 


A II  rights  reserved 


PREFACE 

IN  accordance  with  the  object  of  this  series  of 
Primers,  I  have  endeavoured,  in  this  short  intro- 
duction to  Geology,  to  explain  some  important 
geological  principles  which  are  often  omitted  from 
works  of  this  size,  owing  to  the  difficulty  of  pre- 
senting these  subjects  in  so  short  a  space  and 
in  simple  language.  I  have  been  encouraged 
in  this  course  by  the  rapid  spread  of  scientific 
knowledge  and  interest  during  recent  years.  It 
seems  now  possible  to  introduce  into  elementary 
text-books  questions  that  formerly  were  neces- 
sarily omitted.  More  space  is  given  to  the 
materials  of  the  earth  than  to  the  geographical 
processes  that  affect  them;  for  though  physical 
geology  is  one  of  the  most  popular  branches  of  the 
science,  the  study  of  the  rocks  of  the  earth's  crust 
is  the  essential  basis  of  geology.  I  hope  that  the 
sketch  of  this  subject  will  be  found  sufficiently 
popular  to  be  intelligible  to  the  general  reader  and 
at  the  same  time  useful  to  elementary  students. 
Those  who  have  no  previous  knowledge  of  chemistry 
would  do  well  to  read  Sir  W.  Tilden's  primer  in 
this  series  in  order  to  understand  the  chemical 
processes  concerned  in  the  formation  of  rocks. 

I  am  indebted  to  Mr.  J.  W.  Reoch  for  the  loan 
of  the  photographs  of  Figs,  n,  12,  16,  26,  and  27. 

J.  W.  G. 


CONTENTS 


I' AGE 

INTRODUCTION            ......  9 

PART  I 

CHAP. 

I.  The  Early  History  of  the  Earth  .          .          .11 

PART  II 

THE    MATERIALS    OF    WHICH    THE    EARTH    IS    MADE 

II.  The  Materials  of  the  Earth's  Crust       ...  20 

III.  The  Primary  or  Igneous  Rocks  and  the  Minerals 

of  which  they  are  composed          ...  22 

IV.  The  Classification  of  the  Primary  Rocks        .          .  29 
V.  The  Secondary  or  Stratified  Rocks       ...  37 

VI.  Sedimentary  Deposits          .....  41 

VII.  Chemically  Formed  Rocks            ....  43 

VIII.  Organically  Formed  Rocks           .          .          ...  50 

IX.  The  Metamorphic  Rocks 58 

PART  III 

PHYSICAL    GEOLOGY 

X.  The  Wearing  away  of  the  Land  61 

XI.  How  Secondary  Rocks  are  deposited   ...  77 

XII.  The  Arrangement  of  Rocks  in  the  Field        .          .  87 

XIII.  The  Disturbances  in  the  Rocks  of  the  Crust            .  94 

XIV.  Volcanoes,  Earthquakes,  and  Earth  Movements    .  99 

PART  IV 

HISTORICAL    GEOLOGY 

XV.  The  Study  of  Fossils  ...  .104 

XVI.  Summary  of  Historical  Geology  .          .          .117 

GLOSSARY          .          .          .          .          .          .  133 

INDEX      .          .                    .....  137 


GEOLOGY 


INTRODUCTION 

GEOLOGY  is  the  science  which  investigates  the 
substance,  structure,  and  history  of  the  earth. 
The  materials  of  which  the  earth  is  made  supply 
the  records  for  its  history;  but  these  records, 
enduring  and  impartial  as  they  may  be,  are  not 
always  easy  to  read,  and  it  is  the  duty  of  the  geo- 
logist to  decipher  them  by  the  light  of  processes 
which  can  be  seen  in  operation  at  the  present  time. 
The  chief  material  studied  by  the  geologist  is 
furnished  by  the  rocks  composing  the  crust  of  the 
earth,  and  their  investigation  requires  a  know- 
ledge of  several  branches  of  science.  Thus  rocks 
consist  of  minerals,  which  must  be  studied  with 
the  help  of  physics  and  chemistry;  and  they 
exhibit  structures  whose  origin  can  only  be  ex- 
plained by  the  physical  geographer,  as  he  watches 
the  formation  of  similar  rocks  to-day.  Many 
rocks  contain  the  remains  of  animals  and  plants, 
and  their  nature  must  be  determined  by  the  aid 
of  zoology  and  botany.  Other  rocks  have  been 
formed  deep  beneath  the  surface,  far  deeper  than 
man  can  go,  but  we  are  beginning  to  learn  more  of 
the  nature  of  the  earth's  hidden  substance  from 
the  study  of  earthquakes  and  the  investigation 
of  radium.  Occasionally  lumps  of  mineral,  found 
upon  the  earth,  have  fallen  on  to  it  from  the  sky; 
the  astronomer  must  explain  their  origin,  while 
learning  from  their  mineral  composition  some- 
9 


io  GEOLOGY 

thing  about  those  other  worlds  which  are  the  object 
of  his  special  research. 

Hence,  geology  is  largely  dependent  on  other 
sciences  for  its  advance:  it  depends  on  the  help 
of  chemist,  physicist,  geographer,  zoologist, 
botanist,  and  astronomer.  The  geologist  applies 
their  several  discoveries  to  the  elucidation  of 
his  own  problem — the  examination  of  the  sub- 
stance, structure,  and  history  of  the  earth.  His 
work  is  therefore  varied,  and  the  mastery  of  any 
one  branch  of  geology  requires  a  knowledge  of 
the  various  sciences  connected  with  that  branch. 
Hence  geologists  usually  find  themselves  obliged 
to  confine  their  activity  to  some  one  branch  of 
geology;  but  their  efforts  in  a  special  direction 
will  only  yield  the  fullest  results  when  based 
upon  an  understanding  of  geology  as  a  whole. 

An  elementary  statement  of  the  main  con- 
clusions of  geology  may  be  understood  without 
reference  to  the  methods  by  which  they  have 
been  gained,  though  it  will  always  be  necessary 
to  learn  the  technical  terms  used  for  new  ideas 
and  specific  materials. 

Though  geological  research  usually  requires 
some  previous  training  in  other  branches  of 
science,  yet  valuable  original  work  may  be  done 
by  those  who,  without  any  elaborate  equipment 
or  special  knowledge,  will  carefully  observe  the 
rocks,  collect  the  fossils,  or  ponder  over  the 
structure  of  the  country  around  them. 


PART  I 

CHAPTER  I 

THE  EARLY  HISTORY  OF  THE  EARTH 

"  IN  the  beginning  the  earth  was  without  form, 
and  void,  and  darkness  was  on  the  face  of  the 
deep." 

The  story  of  the  earth,  from  its  original  form- 
less state  and  primeval  darkness  till  the  beginning 
of  written  history,  is  the  subject-matter  of  geology. 
The  story  is  imperishable,  for  it  is  inscribed  on 
the  rocks  of  the  earth's  crust,  and  their  evidence 
is  less  liable  to  error  than  those  human  records 
which  are  subject  to  the  distortions  of  prejudice 
and  misapprehension. 

Before  the  records  of  rocks  can  be  read  their 
language  must  be  learnt.  The  particles  of  the 
earth's  crust  are  the  letters  of  the  story,  and 
they  all  help  to  tell  what  was  happening  on  the 
earth  at  the  time  they  reached  their  present 
resting-place.  Some  of  the  materials  are  easily 
examined  and  their  interpretation  is  clear. 
Gravel-pits,  for  instance,  occur  in  the  neighbour- 
hood of  most  large  towns,  and  the  pebbles  of  the 
gravel  tell  clearly  of  past  changes  in  the  earth, 
for  they  were  once  part  of  rock  masses  now  partly 
or  wholly  destroyed. 

ii 


12  GEOLOGY 

Again,  the  materials  of  the  earth's  crust  are 
exposed  along  the  cliffs  of  the  sea-shore,  on 
river  banks,  and  as  ribs  of  hard  rock  jutting  out 
from  the  hill-side.  These  rocks  may  also  contain 
petrified  shells,  which  will  indicate  when  the  rocks 
were  laid  down,  and  whether  in  fresh  water  or 
beneath  the  sea. 

Some   of   the   materials   of   the   earth's    crust, 


Fig.  i. 

The  Position  of  the  Nebula  in  the  Sword  of  Orion.  The  three 
small  stars  represent  Orion's  Belt:  the  nebula  (n)  surrounds  the 
middle  star  (0)  in  the  Sword.  The  stars  are  arranged  as  they 
would  appear  in  Britain  at  9  p.m.  in  the  latter  part  of  January. 

however,  are  deeply  buried,  and  only  accessible 
in  mines,  wells,  or  bore-holes.  Others  are  beyond 
our  direct  reach,  and  must  be  investigated  by 
indirect  methods;  but  such  evidence  as  they 
afford,  though  more  difficult  to  obtain,  may  be 
as  clear  and  instructive  as  that  of  material  which 
can  be  actually  obtained  and  handled. 

The  state  of  the  earth  was  originally  very 
different  from  its  present  condition;  for,  though 
it  may  have  contained  much  the  same  amount  of 
material  as  it  does  now,  it  occupied  a  far  larger 
space,  and  was  spread  out  in  a  loose  cloud  of 
scattered  bodies.  On  a  clear  starlight  night 


EARLY  HISTORY  OF  THE  EARTH  13 

look  at  the  three  stars  known  as  Orion's  Belt, 
and  below  them,  if  the  observer  be  in  the  northern 
hemisphere,  will  be  seen  a  curved  line  of  three 
stars  —  the  Sword  of  Orion  (Fig.  i).  Examine 
the  middle  star  closely,  if  possible  through  a 


Fig.  2. 

The  Great  Nebula  in  Orion. 
(From  a  photograph  taken  by  the  Yerkes  Observatory.) 

telescope  or  pair  of  opera  glasses;  but  even  with 
the  naked  eye  that  star  will  appear  to  be  more 
misty  and  to  have  less  definite  borders  than  its 
neighbours.  It  is  a  nebula,  and  its  condition,  as 
revealed  by  a  powerful  telescope,  is  illustrated 
by  Fig.  2,  while  another  nebula  (in  the  con- 
stellation of  the  Hunting  Dogs),  which  has  a 
spiral  structure,  is  illustrated  by  Fig.  3. 

The  earth  was  once    a   hot    nebula,    but    the 
condition    of    its    material    then    is    uncertain. 


14  GEOLOGY 

Opinions  differ  as  to  whether  it  consisted  of  a 
vast  cloud  of  incandescent  gas,  which  condensed 
as  it  cooled,  or  whether  its  earliest  condition  was 
a  swarm  of  metallic  masses,  composed  mainly 
of  iron  and  nickel,  which  slowly  united  into  a 
compact  whole.  This  union  would  have  been 
accompanied  by  so  great  an  increase  in  tempera- 
ture that  the  separate  iron  bodies  would  have 
been  fused  into  one  mass. 

The  arguments  for  and  against  a  gaseous, 
or  a  solid  swarm-like  beginning  of  the  earth 
are  largely  derived  from  astronomy;  and  the 
question  mainly  concerns  the  geologist  from  its 
bearing  on  the  geographical  conditions  at  the 
dawn  of  geological  history.  For  if  the  earth  had 
been  once  composed  of  white-hot  gas,  its  first 
known  climate  should  have  been  much  hotter 
than  that  of  the  present  time,  and  the  tempera- 
ture should  have  become  slowly  cooler.  If,  on 
the  contrary,  the  earth  began  as  a  swarm  of 
solid  bodies,  it  need  never,  as  a  whole,  have  been 
much  hotter  than  it  is  now. 

Geology  shows  that  the  earliest  known  climate 
was  very  similar  to  that  of  to-day.  The  rain- 
drops were  of  the  same  size,  and  the  wind  was 
of  the  same  force,  while,  instead  of  the  earth  being 
warmer  than  now,  icebergs  floated  nearer  the 
tropics  in  the  primeval  than  in  the  existing  seas. 

Geological  records  give  no  certain  evidence  of 
any  period  in  the  earth's  past,  when  the  climate 
of  the  whole  earth  was  much  hotter  than  it  is  to- 
day, nor  do  they  afford  any  indication  of  a  time 
when  wind,  rain,  tide,  and  other  geographical  forces 
were  of  much  greater  power  than  they  are  now. 

The  distribution  of  physical  activity  on  the 
earth  has,  however,  often  been  different,  as  may 


EARLY  HISTORY  OF  THE  EARTH     15 

be  seen  when  we  study  volcanic  rocks.  The  only 
active  volcanoes  in  Europe  are  Vesuvius  and 
Etna  on  the  Continent,  as  well  as  the  volcanic 
islands  near  Sicily,  those  in  the  Grecian  Archi- 


Fig.  3- 

The  Spiral  Nebula  in  the  Constellation  of  the  Hunting  Dogs. 

(From  a  photograph  by  the  Yerkes  Observatory.) 
The  evidence  of  the  spectroscope  indicates  that  the  material 
of  this  nebula  is  solid. 

pelago,  and  the  two  northern  islands  of  Iceland 
and  Jan  Mayen.  Going  to  earlier  times,  we  find 
records  of  probable  volcanic  eruptions  in  central 
France  as  late  as  the  fifth  century  A.D.  ;  extinct 
volcanoes  are  to  be  found  in  nearly  all  European 
countries,  and  not  only  were  volcanoes  once 
active  in  many  places  where  they  are  now  ex- 


16  GEOLOGY 

tinct,  but  at  certain  earlier  periods  volcanic  action 
was  far  more  widespread  than  it  has  been  during 
the  experience  of  man. 

Again,  there  are  no  glaciers  at  present  in  the 
British  Isles;  but  the  mountains  of  Wales, 
Scotland,  and  the  north  of  England  were  once 
snow  covered  all  the  year  round,  and  rivers  of  ice 
flowed  down  the  mountain  slopes  and  covered 
parts  of  the  British  lowlands. 

It  is  clear,  therefore,  that  there  have  been 
great  changes  in  the  geography  of  the  world  in 
past  times;  and  the  present  condition  of  the 
earth's  surface  is  the  result  of  an  unceasing 
struggle  between  the  forces  of  waste  and  the 
forces  of  repair.  We  have  only  to  look  attentively 
at  the  district  in  which  we  live,  to  see  that  these 
forces  are  still  at  work. 

In  the  city  we  can  observe  the  decay — the 
often  deplorably  rapid  decay — of  stones  used  for 
building;  and  it  is  rare  in  a  churchyard  to  find 
an  intelligible  inscription  on  a  tombstone  more 
than  a  century  or  two  old,  unless  it  has  been 
recut.  In  the  country  we  notice  the  crumbling 
of  rocks  before  the  weather,  the  deepening  of 
lanes  and  roads  by  the  wear  of  traffic  and  the 
wash  of  rain.  Along  the  coast  we  observe  the 
cutting  back  of  cliffs  by  the  attack  of  the  sea. 
How  unceasing  and  rapid  is  the  waste  of  the 
land  may  be  judged  by  looking  at  an  ordinary 
lowland  river;  the  water  is  usually  of  a  brownish 
colour,  owing  to  the  quantity  of  floating  mud  it 
contains — mud  derived  from  the  wearing  away 
of  the  land.  The  amount  of  material  thus  re- 
moved by  rivers  is  enormous.  Every  year  the 
Thames  carries  to  the  sea  over  500,000  tons  of 
mineral  matter.  The  load  of  material  is  dropped 


EARLY  HISTORY  OF  THE  EARTH     17 

by  the  river  on  its  bed  or  at  its  mouth  as  a  delta, 
or  spread  over  the  floor  of  a  lake  or  of  the  sea. 

Thus  land  is  being  formed  at  one  place  with 
the  materials  derived  from  the  destruction  of 
land  at  another  place;  and  the  most  elementary 
problems  in  geology  are  concerned  with  these 
processes,  which  are  still  moulding  the  surface 
of  the  earth;  but  to  understand  these  processes 
of  rock  destruction  and  rock  formation,  it  is 
necessary  first  to  consider  the  nature  of  the 
materials  of  which  the  earth  is  made. 


PART   II 

THE  MATERIALS  OF  WHICH  THE 
EARTH  IS  MADE 

IN  whichever  manner  it  may  have  originated, 
the  earth  as  a  whole  may  be  regarded  as  a 
ball,  consisting  of  a  central  core,  covered  by 
three  successive  skins  or  layers,  which  form  the 
main  field  of  geological  research. 

The  vast,  inaccessible,  central  mass,  which 
constitutes  by  far  the  largest  part  of  our  earth, 
is  hidden  from  our  observation  and  shielded  from 
our  experiments. 

Where  sense  fails  us  mind  alone  can  penetrate, 
and  our  knowledge  concerning  the  central  core 
of  the  earth  is  almost  entirely  a  matter  of  in- 
ference and  analogy. 

Two  words  have  been  employed  as  names  for 
this  central  core;  it  has  been  called  the  centro- 
sphere  on  account  of  its  central  position,  and 
the  barysphere  on  account  of  its  weight. 
Barus  is  the  Greek  word  for  "  heavy,"  and  when 
we  use  the  term  "barysphere"  we  are  affirm- 
ing the  one  definite  conclusion  that  has  been 
arrived  at  as  to  the  nature  of  the  earth's  central 
mass,  and  that  is  its  exceptional  weight.  The 
materials  composing  the  core  of  the  earth 
are  more  than  twice  as  heavy  as  the  materials 
composing  the  superincumbent  outer  layers. 
18 


MATERIALS  19 

The    term    "  barysphere "    is    therefore    appro- 
priate to  the  unexplored  core  of  our  earth. 

The  barysphere  is  surrounded  by  the  litho- 
sphere  (Greek  lithos,  a  rock),  the  rocky  crust  of 
the  earth.  Resting  upon  the  lithosphere  is  the 
hydrosphere,  which  includes  all  the  waters  on  or 
in  the  earth's  surface;  and  the  outermost  layer 
is  the  atmosphere,  the  gaseous  envelope  which 
completely  surrounds  the  earth;  thus  recalling 
the  four  elements  of  the  ancients — earth,  air, 
fire,  and  water. 

The  chief  materials  with  which  the  geologist 
has  to  do  are  the  rocks  of  the  outer  layers  of  the 
lithosphere. 

The  term  "  rock  "  appears  to  be  derived  from 
the  same  root  as  the  word  "  crag,"  and  to  mean  a 
firm,  stony  material.  It  is  used  in  that  sense 
in  current  English,  but  by  a  widely  accepted 
convention  some  geologists,  for  the  sake  of  con- 
venience, apply  it  to  all  the  materials  of  the 
earth's  crust,  and  thus  regard  loose  beds  of  sand 
and  soil,  ice,  and  even  water,  as  rock.  This  use 
of  the  word  ignores  the  essential  idea  of  the 
term,  as  expressed  in  the  comparison  "  firm  as  a 
rock."  It  will  therefore  be  clearer  to  use  the 
term  "  rock  "  in  its  primitive  and  ordinary  sense, 
taking  the  word  "  crust "  to  describe  the  whole 
covering  of  the  barysphere,  which  we  roughly 
divide  according  to  its  solid,  liquid,  or  gaseous 
components  into  lithosphere,  hydrosphere,  and 
atmosphere. 


20  GEOLOGY 

CHAPTER  II 

THE   MATERIALS   OF   THE    EARTH'S   CRUST 

THE  materials  of  the  lithosphere,  the  solid  layer 
of  the  earth's  crust,  belong  to  two  main  divisions, 
and  are  classed  as  primary  and  secondary  rocks. 
Primary  rocks  are  those  which  have  been  formed 
by  direct  consolidation  from  molten  material. 
Secondary  rocks  are  those  which  have  been 
formed  by  the  destruction  of  the  primary  rocks 
and  the  redeposition  of  their  material. 

The  primary  and  secondary  classes  of  rocks 
may  be  separated  by  several  well-marked  char- 
acters. Primary  rocks  are  composed  mainly  of 
crystalline  materials,  or  of  a  natural  glass ; 
secondary  rocks  are  mostly  formed  of  fragmentary 
grains,  and  they  are  therefore  said  to  be  "  clastic  " 
(Greek  klastos,  broken  in  pieces). 

A  second  important  distinction  is  that  the 
primary  rocks  frequently  occur  in  large  masses, 
which  are  often  uniform  in  character  for  a  great 
thickness;  whereas  the  secondary  rocks  are 
formed  of  a  succession  of  layers  or  "  strata  "  that 
have  been  deposited  one  over  the  other,  and 
though  these  layers  may  be  uniform  in  character 
over  wide  sheets,  they  usually  occur  in  layers 
which  are  thin  in  comparison  to  their  extent. 

The  primary  rocks  not  being  formed  in  layers, 
are  said  to  be  "  unstratified,"  while  the  secondary 
rocks  are  termed  "  stratified." 

Some  of  the  secondary  rocks  are  not  com- 
posed of  fragments  of  primary  rocks,  but  they 
are  formed;  from  the  skeletons  and  shells  of  the 
animals  and  plants  which  lived  while  the  rock 
was  being  formed.  The  remains  of  animals  and 


MATERIALS  OF  THE  EARTH'S  CRUST    21 

plants  buried  in  the  earth's  crust  are  known  as 
fossils,  and  they  may  occur  either  as  a  small 
percentage  of  a  rock  or  as  the  main  bulk  of  it. 
Skeletons  and  shells  consist  of  material  which 
has  been  derived  from  the  primary  rocks;  so 
fossils  are  regarded  as  secondary  rock  consti- 
tuents. Hence,  the  presence  or  absence  of  fossils 
is  a  further  distinction  between  the.  primary  and 
secondary  rocks.  For  as  primary  rocks  were  formed 
under  conditions  too  hot  and  usually  too  deeply 
buried  beneath  the  earth's  surface  for  the  exist- 
ence of  animals  or  plants,  they  contain  no  fossils ; 
whereas  most  secondary  rocks  contain  fossil 
remains  of  the  organisms  living  at  the  place 
where  the  rock  was  being  formed,  or  which  have 
been  washed  into  it  from  some  older  rock. 
Primary  rocks  are,  therefore,  unfossiliferous,  and 
secondary  rocks  are  usually  fossiliferous. 

Primary  rocks,  having  been  formed  under  the 
influence  of  intense  heat,  are  described  as 
igneous  rocks;  while  as  a  large  proportion  of  the 
secondary  rocks  have  been  laid  down  under  water, 
they  are  called  aqueous  rocks;  and  the  rest, 
which  were  formed  on  land,  are  called  aeolian  or 
subaerial  rocks. 

These  differences  are  summarised  in  the  follow- 
ing statement — 

Primary  rocks  are  Igneous,  Unstratified,  Crys- 
talline or  Glassy,  Unfossiliferous. 

Secondary  rocks  are  Aqueous  or  Subaerial, 
Stratified,  Fragmentary,  Fossiliferous. 


22  GEOLOGY 


CHAPTER  III 

THE    PRIMARY    OR    IGNEOUS    ROCKS    AND    THE 
MINERALS   OF   WHICH   THEY   ARE   COMPOSED 

THE  primary  or  igneous  rocks  are  divided  into 
two  main  series — those  that  consolidated  at  a 
great  depth  beneath  the  surface,  and  those  that 
consolidated  at  or  near  the  surface.  The  deep- 
seated  igneous  rocks  are  called  "  plutonic," 
after  Pluto,  the  god  of  the  infernal  regions;  the 
superficial  igneous  rocks  are  called  the  "  volcanic," 
after  Vulcan,  who  had  his  workshop  near  the 
earth's  surface.  Intermediate  between  these  two 
groups  are  the  rocks  that  have  solidified  beneath 
but  near  the  surface,  in  fissures  or  pipes  filled  by 
molten  material  that  has  been  forced  into  them 
from  below. 

The  factor  that  determines  whether  liquid  rock 
material  forms  a  plutonic  or  a  volcanic  rock  is 
the  pressure  at  which  it  solidifies.  If  a  mass  of 
molten  rock  be  cooled  quickly  and  at  slight 
pressure,  it  solidifies  as  a  glass;  but  if  it  cool 
slowly  and  under  heavy  pressure,  it  solidifies  as 
a  stony  mass.  This  fact  is  exemplified  in  the 
preparation  of  the  glass  known  as  Chance's 
artificial  stone.  Basalt  is  melted  and  poured 
into  moulds,  and  when  it  cools  quickly,  it  solidi- 
fies as  a  glass;  but  if  it  cool  slowly,  it  forms  into 
a  stony  mass  composed  of  a  mixture  of  glassy 
and  crystalline  materials.  The  dull,  earthy- 
looking  basalt  is  composed  of  a  mixture  of 
minerals,  which  are  not  glassy  but  crystalline, 
though  the  crystals  may  be  too  small  to  dis- 
tinguish by  the  unaided  eye;  but  if  examined 
by  a  powerful  lens,  or  still  more  clearly  if  a  thin 


ROCKS  AND  MINERALS 


slice  of  the  rock  be  examined  with  the  aid  of  the 
microscope,  it  is  seen  to  be  composed  of  a  mixture 
of  materials,  of  which  most,  or  perhaps  all,  are 
crystalline. 

The  essential  difference  between  a  crystalline 
and  a  glassy  substance  is  in  their  minute  internal 
structure.  In  a  glass  the  particles  are  arranged 
irregularly,  whereas  in  a  crystalline  substance 
the  particles  are  arranged  regularly,  so  that  a 


Fig.  4. 
Three  Sections  of  Rocks. 

a.  A  Granite.    The  large  angular  crystals  are  the  felspar ;  the 
black  is  the  mica;   the  rest  is  mainly  quartz  and  felspar. 

b.  A   Liparite.     The  large  clear  crystals   are  the   corroded 
felspars:    the  lath-shaped  crystals  the  mica:    the  rest  is  the 
glassy  base. 

c.  A  Grit.     A  sedimentary  rock  mainly  of  grains  of  quartz 
(the  clear  grains)  and  felspar  (the  grains  with  the  crowded  dots). 

given  number  of  particles  occupies  less  space  in 
the  crystalline  than  in  the  glassy  condition. 
Hence,  if  a  molten  material  be  solidified  under 
such  heavy  pressure  that  it  is  compressed  into 
the  smallest  possible  space,  it  must  solidify  as  a 
crystal  and  not  as  a  glass. 

This  fact  may  be  illustrated  in  various  ways. 
Thus  a  granite  is  a  rock  which  has  cooled  under 
such  enormous  pressure  that  all  its  materials 
are  crystalline ;  but  if  a  piece  of  granite  be  melted 
and  cooled  quickly  free  from  pressure,  it  will 


24  GEOLOGY 

solidify  as  a  glass.  This  granite  glass  is  lighter, 
bulk  for  bulk,  than  the  crystalline  granite;  thus 
a  cubic  foot  of  granite  weighs  about  one  hundred 
and  seventy  pounds;  but  a  cubic  foot  of  the 
glass  formed  by  the  melting  and  quick  cooling  of 
this  granite  will  weigh  only  one  hundred  and 
fifty  pounds.  A  cubic  foot  of  basalt  weighs 
about  one  hundred  and  eighty  pounds,  and  of 
basalt  glass  only  one  hundred  and  sixty -four 
pounds. 

The  presence  or  absence  of  glass  in  a  primary 
rock  therefore  affords  a  measure  of  the  depth 
and  pressure  under  which  the  rock  solidified.  A 
rock  consisting  wholly  of  glass  must  have  been 
formed  on  the  earth's  surface,  and  under  slight 
pressure.  A  rock  formed  wholly  of  crystalline 
material,  on  the  other  hand,  has  usually  consoli- 
dated under  heavy  pressure.  Hence  plutonic 
rocks  are  wholly  crystalline,  or  "  holocrystalline." 
Volcanic  rocks  may  be  wholly  glassy  like  obsidian  ; 
or  they  may  be  composed  of  a  mixture  of  glassy 
and  crystalline  material,  and  then  they  are 
partly  crystalline,  or  "  merocrystalline." 

The  primary  rocks  of  the  earth's  crust  are 
mostly  composed  of  a  comparatively  few  distinct 
kinds  of  crystalline  material.  Each  kind  of  these 
crystalline  substances  is  known  as  a  Mineral 
Species;  and  an  igneous  rock  is  usually  a  mixture 
of  two,  three,  or  more  of  these  mineral  species. 
The  chemical  composition  of  most  mineral  species 
is  more  or  less  constant;  so  that  by  determining 
the  various  mineral  species  present  in  a  rock, 
and  their  relative  proportions,  the  total  composi- 
tion of  the  rock  can  be  determined.  The  identi- 
fication of  minerals  in  rocks  is  most  easy  in  those 
which  are  composed  of  crystals  large  enough  to 


ROCKS  AND  MINERALS  25 

be  recognised  by  the  naked  eye,  and  as  this  is 
the  case  in  the  plutonic  rocks,  it  is  most  con- 
venient to  study  them  first. 

The  mineral  species  of  which  rocks  are  composed 
belong  to  the  following  groups — 

1.  Quartz. — This    mineral  —  known    as    rock- 
crystal    when    found    in    the    clear    transparent 
crystals   used   for  "  pebble  "  glasses — consists  of 
the    element    silicon    combined    with    the    gas 
oxygen,    forming    the    compound    silica    (SiO2). 
Quartz   sometimes   occurs    as   six-sided   crystals, 
composed  typically  of  a  six-sided  prism  with  a 
six-sided  pyramid  at  each  end;    but  it  is  usually 
found  as  irregular  grains.     It  is  so  hard  that  it 
cannot  be  scratched  by  a  knife,  it  has  a  glassy  or 
milky-white  appearance,  and  it  is  not  dissolved 
by  water  or  most  acids. 

2.  Felspar. — The  felspars  are  a  group  of  mineral 
species  which  agree  in  many  respects  in  form, 
properties,    and   composition.     They   all   contain 
silica  (SiO,)  and  alumina  (A1,O3);    some  of   them 
contain  in  addition  the  alkalies  potash  (K2O)  or 
soda  (Na^O),  and  others  contain  the  earth,  lime 
(CaO),  or  mixtures  of  lime  and  soda.      The  fel- 
spars are  of  two  kinds :  the  basic  l  felspars,  which 
contain  more  lime  than  alkali ;    and  the  acid 1 
felspars,  those  containing  more  alkali  than  lime,  and 
a  higher  percentage  of  silica  than  the  basic  felspars. 

3.  Felspathoids. — a    group    of    mineral    species 
which  may  sometimes    replace    the    felspars    in 
igneous    rocks;     the    chief    species    are    leucite, 
sodalite,  and  nepheline. 

4.  Micas — a  group   of  mineral   species,   which 

1  For  the  definition  of  acid  and  base,  see  Sir  W.  Tilden's 
primer  on  Chemistry,  p.  91.  A  base  is  a  material  the 
whole  of  which  will  combine  with  an  acid, 


26  GEOLOGY 

all  crystallise  in  flat  plates  or  scales,  and  have 
the  quality  of  breaking  into  thin  flat  elastic 
scales  or  "  cleavage  flakes."  Ordinary  white 
mica,  which  is  used  for  lamp  shades,  is  composed 
of  silica,  alumina,  and  potash;  the  black  and 
brown  micas  contain  silica,  iron,  and  magnesia, 
and  the  presence  of  the  iron  makes  them  heavier 
and  darker  in  hue  than  the  white  micas. 

5.  Ferromagnesian     Minerals  —  a     group     of 
mineral  species  partly  formed  of  iron  and  mag- 
nesium.    These   two   constituents   are  known   as 
the  "  femic  "  constituents,  from  the  combination 
of   the   first   letters   of   their   chemical   symbols, 
Fe  and  Mg.     The  chief  of  the  femic  minerals  are 
the   amphiboles  (e.g.  hornblende),  the  pyroxenes 
(e.g.  augite),  and  olivine.    The  dark  colour  of  many 
igneous   rocks   is   due   to  the   abundance   of  the 
femic  minerals.     Owing  to  the  iron  present  these 
femic  minerals  are  usually  dark  in  colour,   and 
heavier,  bulk  for  bulk,  than  minerals  richer  in  silica. 

6.  Accessory    Minerals. — Many    igneous    rocks 
contain    small    grains    and    crystals    of    various 
oxides   of   metals,   such   as   magnetite,    oxide   of 
iron;    also  crystals  of  apatite,  phosphate  of  lime, 
and  of  zircon  and  other  minerals. 

The  leading  difference  between  the  members 
of  these  six  groups  of  minerals  is  the  varying 
amount  of  the  "  acid  "  or  silica  present,  in  rela- 
tion to  the  oxides  of  the  metals  and  other  "  basic  " 
constituents.  Thus  quartz  (SiOJ  consists  wholly 
of  silica,  and  there  is  no  basic  constituent  present  ; 
while  passing  through  the  other  groups,  there 
is  a  steady  increase  in  the  amount  of  "  base  " 
present,  till  magnetite  (Fe3O4),  a  member  of  the 
last  group,  consists  wholly  of  base,  and  contains 
none  of  the  acid. 


ROCKS  AND  MINERALS  27 

Hence  igneous  rocks  rich  in  quartz  contain 
more  acid  than  rocks  that  are  without  quartz, 
and  are  rich  in  iron  and  magnesia. 

The  plutonic  rocks  are  divided  into  various 
classes  according  to  their  composition;  those 
which  contain  much  quartz — the  acid  constituent 
— are  the  acid  rocks,  such  as  granite.  From 
this  extreme  on  the  one  side,  to  the  ultra-basic 
group  on  the  other,  there  is  a  gradual  passage, 
marked  by  a  decrease  in  the  amount  of  silica 
and  alkalies,  and  an  increase  in  the  proportion 
of  basic  materials,  the  iron,  magnesia,  and  lime, 
to  the  ultra-basic  group.  The  rock  series  is  con- 
tinuous, but  it  is  divided  for  convenience  into  five 
main  classes.  The  chief  rock  in  each  class  is  as 
follows — 

Rock  Constituents 

Granite  Quartz,  acid  felspar,  and  mica 

Syenite         .         Acid  felspar  and  hornblende 
Diorite  Basic     ,, 

Gabbro  „        ,,         „     altered   augite 

Peridotite  No    felspar;     the    rock    is    often 

mostly  composed  of  olivine 

Each  of  these  rocks  is  composed  wholly  of  crystal- 
line materials,  and  the  minerals  are  usually  so 
large  in  grain  that  they  can  be  recognised  by 
the  unaided  eye. 

These  plutonic  rocks  generally  occur  in  large 
thick  masses,  which  may  be  many  square  miles 
in  area,  and  are  of  great  and  usually  unknown 
thickness.  Thus  the  whole  of  Dartmoor  consists 
of  one  great  block  of  granite  200  square  miles  in 
area.  The  rock  on  the  edge  of  such  a  mass 
cools  more  quickly  than  that  in  the  middle,  and 
thus  the  grain  of  the  rock  is  finer  on  the  margin. 
Tongues  or  sheets  of  granite  may  project  from  the 
mass  into  the  surrounding  rocks;  these  tongues 


28  GEOLOGY 

or  sheets  are  known  as  dykes,  and  their  rocks  may 
be  composed  either  entirely  of  crystalline  material, 
or  of  a  mixture  of  crystals  and  glassy  rock  material. 

Some  of  the  fissures  into  which  these  dyke 
rocks  were  forced  from  below  may  have  reached 
up  to  the  surface,  and  their  material  may  then  have 
been  discharged  in  volcanic  eruptions. 

If  the  plutonic  rock  be  a  granite,  then  the  lava 
which  pours  over  the  surface  through  a  volcanic 
vent  will  be  a  rock  light  in  weight,  pale  in  colour, 
and  rich  in  silica;  and  as  lavas  of  this  type  have 
been  discharged  abundantly  from  the  volcanoes 
of  the  Lipari  Isles  near  Sicily,  these  lavas  are 
called  liparites.  If  a  dyke  rock  from  a  gabbro 
mass  reach  the  surface,  the  material  will  flow  as 
a  lava  which  is  darker  in  colour  and  heavier  than 
the  liparite,  and  it  will  not  contain  any  quartz; 
it  will  be  a  basalt.  Each  kind  of  plutonic  rock 
has  its  corresponding  lava. 

The  lava  equivalent  of  a  granite  is  a  liparite 
,,  „  syenite        ,,    trachyte 

,,  „  diorite        ,,    andesite 

,,  ,,  gabbro       ,,    basalt  or  doled te 

,,  ,,  peridotite  ,,    limburgite 

There  are  also  intermediate  varieties  of  plutonic 
rocks,  each  with  its  corresponding  lava. 

The  various  types  of  plutonic  rocks  are  not 
separated  by  sharply  defined  boundaries;  each 
rock  grades  into  the  varieties  on  either  side  of 
it,  and  there  is  a  continuous  series  from  the  most 
acid  to  the  most  basic  rocks.  Indeed,  it  is  possible 
that  all  the  rocks  in  the  series  may  be  variations 
from  one  rock  material,  and  that  the  diversity 
of  products  from  the  same  source  is  due  to  the 
fact  that  the  different  minerals  crystallise  out  of 
a  molten  mass  in  a  regular  succession, 


CLASSIFICATION  OF  PRIMARY  ROCKS     29 

The  basic  minerals  crystallise  before  the  more 
acid.  The  very  basic  accessory  minerals,  includ- 
ing the  metallic  oxides,  crystallise  first;  the 
ferromagnesian  minerals,  being  less  basic,  follow; 
then  come  the  still  less  basic  felspathoids  and 
felspars;  and  finally  the  baseless  quartz.  The 
sequence  of  the  minerals  is,  therefore,  in  the 
order  of  decreasing  baseness  or  basicity.  As  the 
more  basic  materials  are  removed  from  the 
molten  rock  by  crystallisation,  the  molten  residue 
gradually  becomes  more  and  more  acid.  More- 
over, as  the  basic  minerals  are  developed,  they 
tend  to  collect  either  on  the  bottom  or  on  the 
cooler  margins  of  the  solidifying  rock  mass. 
Hence  by  this  process,  the  collection  or  segrega- 
tion of  the  basic  material,  the  mass  loses  its 
originally  uniform  composition,  and  divides  into 
two  parts ;  one  of  them  may  have  the  composition 
of  a  granite,  and  the  other  of  a  gabbro;  and 
they  may  be  discharged  at  the  surface  as  acid 
and  basic  lavas.  This  gradual  separation  of  the 
acid  and  basic  constituents  is  known  as  differ- 
entiation, and  by  this  process  different  types 
of  igneous  rocks  are  produced  from  one  uniform 
mass  of  molten  rock. 


CHAPTER    IV 

THE   CLASSIFICATION   OF  THE   PRIMARY   ROCKS 

A  CLASSIFICATION  of  rocks,  to  be  of  general  useful- 
ness, must  express  two  facts  about  them:  first, 
the  composition  of  the  rock,  on  which  its  economic 
value  often  depends;  and  second,  the  conditions 
under  which  the  rock  solidified,  since  they  de- 
termine its  structure,  appearance,  and  often  its 


30  GEOLOGY 

colour.  Rocks  of  very  different  composition  may 
resemble  one  another  more  closely  than  rocks 
having  identically  the  same  composition.  Thus 
a  piece  of  granite  may  closely  resemble  in  appear- 
ance pieces  of  the  rocks  known  as  diorite  or  gabbro, 
as  they  may  each  consist  of  a  coarse-grained 
mixture  of  a  white  and  of  a  black  or  dark-green 
mineral,  and  will  all  look  alike  though  they  differ 
widely  in  their  composition.  A  granite,  on  the 
other  hand,  may  have  exactly  the  same  composi- 
tion as  obsidian,  although  they  differ  totally  in 
their  appearance,  for  obsidian  looks  like  black 
bottle  glass. 

The  following  table  shows  the  composition  of 
four  different  rocks — 


•P 

z-felsite, 
iemia. 

•"  2 

|.§ 

5fi 

!m 

&™ 

0§ 

Silica  (SiO2)      . 

73 

74 

73 

74 

Alumina  (Al.,O3) 

14 

13 

14 

14.25 

Oxides  of  iron  (Fe2O3  &  FeO) 

2.5 

2 

1.4 

1.8 

Magnesia  and   lime 

(MgO, 

CaO)    . 

1.9 

i-S 

1.55 

1.4 

Alkalies  —  soda  and 

potash 

(Na20,  K20) 

• 

7-7 

7.6 

8.1 

9 

These  four  rocks,  though  coming  from  distant 
localities,  have  almost  the  same  composition; 
but  the  four  rocks  are  quite  different  in  appear- 
ance. The  granite  is  a  coarse-grained,  grey  rock, 
which  can  be  seen  by  the  naked  eye  to  be  com- 
posed wholly  of  quartz,  felspar,  and  mica.  The 


CLASSIFICATION  OF  PRIMARY  ROCKS     31 

quartz-felsite  and  the  rhyolite  are  finer  grained, 
and  of  a  darker  grey  colour  than  the  granite; 
and  it  is  only  by  the  use  of  a  lens  that  occasional 
fragments  may  be  found  in  them  large  enough 
for  recognition  as  distinct  mineral  species.  The 
last  of  the  four  rocks,  obsidian,  is  a  black,  smooth, 
shiny  glass.  Hence  one  molten  rock  material 
may  produce  several  different  kinds  of  rocks; 
and  the  difference  in  the  product  will  be  due  to 
the  conditions  under  which  it  solidified.  Granite 
is  the  rock  formed  when  acid  rock  material  has 
cooled  very  slowly  under  the  pressure  of  overlaying 
rocks  which  are  miles  in  thickness.  If  the  same 
rock  material  be  forced  up  a  fissure,  and  thus 
cools  more  quickly  and  under  less  pressure,  it 
forms  a  felsite.  If  the  fissure  reach  the  surface 
of  the  earth  and  the  molten  rock  overflow  the 
surface  as  a  lava  stream,  it  forms  a  rhyolite; 
and  if  the  surface  of  the  flow  is  cooled  very 
quickly,  it  forms  a  sheet  of  obsidian.  All  these 
four  rocks  belong,  then,  chemically  to  the  same 
group,  and  are  formed  of  the  same  rock  material 
or  Magma.  Whether  this  magma  solidified  as  a 
granite  which  is  wholly  crystalline,  or  as  obsidian 
which  is  entirely  composed  of  glass,  depends 
simply  on  the  conditions  under  which  it  solidified. 

The  igneous  rocks  may  be  divided  into  five 
groups  according  to  their  chemical  composition. 
All  the  rocks  of  each  of  these  groups  are  produced 
from  one  kind  of  molten  rock  or  magma,  and  the 
exact  kind  of  rock  produced  depends  mainly  on 
the  depth  at  which  their  material  solidifies.  The 
chemical  composition  of  the  typical  rock  of  each 
of  these  five  groups  is  shown  on  the  following 
table— 


GEOLOGY 


tx.  cj  oo  ix. 
co  ci  ci  o 


M    1-1  OC  O 


• 


CLASSIFICATION  OF  PRIMARY  ROCKS     33 

This  table  shows  that  the  chief  differences  in 
composition  between  these  five  types  are  as 
follows:  (i)  A  decrease  in  the  amount  of  silica  as 
the  series  is  followed  from  granite  to  serpentine; 
(2)  a  decrease  in  the  amount  of  the  alkalies, 
potash,  and  soda;  and  (3)  an  increase  in  the 
amount  of  the  bases,  lime,  magnesia,  and  oxide 
of  iron.  As  the  silica  is  the  acid  constituent, 
and  the  lime,  magnesia,  and  the  iron  oxides  are 
the  basic  constituents  in  rocks,  it  follows  that 
the  granite  is  the  more  acid,  and  that  the  gabbro 
and  serpentine  are  the  more  basic. 

Granite  is  the  type  of  the  group  known  as  the 
Acid  Rocks,  and  gabbro  of  the  Basic.  Between 
them  are  two  intermediate  stages,  of  which  the 
syenite,  being  nearer  to  the  granite,  is  known  as 
the  sub-acid,  and  the  diorite,  being  nearer  to  the 
gabbro,  is  known  as  the  sub-basic.  The  serpentine, 
having  even  less  silica  and  more  bases  than 
gabbro,  is  known  as  the  ultra-basic  group. 

The  influence  of  the  chemical  composition  of 
the  rock  taken  in  bulk  naturally  influences  the 
minerals  that  compose  it.  In  the  gabbro  there 
is  so  little  silica  that  it  is  all  used  up  in  com- 
bination with  the  other  constituents,  and  there 
is  none  of  it  left  to  crystallise  as  free  silica  or 
quartz.  In  the  granite,  on  the  other  hand,  the 
amount  of  silica  is  so  large  that,  after  the  other 
constituents  present  have  combined  with  as  much 
silica  as  they  can  use,  there  is  an  excess  left  over 
which  solidifies  as  free  quartz.  The  acid  group 
is,  therefore,  rich  in  quartz;  a  little  quartz  is 
present  in  the  sub-acid,  and  in  both  these  rocks 
the  felspar  that  is  present  is  an  acid  felspar, 
because  it  contains  much  silica.  In  the  gabbro, 
on  the  other  hand,  the  felspar  is  a  basic  felspar, 

c 


34  GEOLOGY 

which  contains  a  smaller  proportion  of  silica  than 
the  felspar  of  a  granite;  and  there  is  usually  also 
present  the  mineral  olivine,  which  contains  a 
smaller  proportion  of  silica  than  any  of  the  con- 
stituents of  granite.  Olivine  cannot  be  formed 
in  the  magma  that  produces  granite,  because  the 
basic  constituents  of  the  olivine  would  combine 
with  some  of  the  extra  silica  and  form  a  more 
siliceous,  i.e.  a  more  acid  mineral  species  than 
olivine. 

Quartz,  then,  is  characteristic  of  the  acid  rocks, 
and  olivine  of  the  basic. 

Five  groups  of  rocks  based  upon  composition 
may  be  arranged  as  in  the  table  opposite. 

In  each  of  these  five  groups  of  rocks  there  is  a 
sub-group,  the  members  of  which  differ  from 
those  of  the  normal  group  by  containing  a  much 
higher  percentage  of  alkalies,  usually  soda.  An 
igneous  rock,  very  rich  in  soda  and  corresponding 
to  the  liparite,  is  known  as  a  keratophyr;  the 
soda-rich  rock  corresponding  to  the  trachytes 
is  a  phonolite ;  that  corresponding  to  an  andesite 
is  a  tephrite ;  and  that  corresponding  to  a  basalt 
is  a  basanite. 

Some  simple  method  is  required  for  determin- 
ing to  which  group  any  particular  rock  belongs. 
To  make  a  full  chemical  analysis  of  a  rock  is  a 
serious  labour,  which  usually  takes  a  skilled 
chemist  about  a  week.  Hence  a  chemical  classi- 
fication of  rocks  would  be  of  little  practical  value 
to  a  geologist  in  the  field,  if  there  were  not  some 
simple  test  by  which  rocks  could  be  assigned 
to  their  respective  groups.  In  the  case  of  a  coarse- 
grained rock  like  a  granite  or  a  gabbro,  the  group 
may  be  determined  by  identifying  the  mineral 


CLASSIFICATION  OF  PRIMARY  ROCKS     35 


H    r_> 

S     IH 


O 


is 


— 

nO   o 
11 


TH  J3          „_,     0>          .00) 

^  ^      ^2  3      'SS'S 


a 


m 


pq 


36  GEOLOGY 

species  of  which  it  is  composed;  but  if  the  rock 
be  very  fine-grained,  the  minerals  can  only  be 
recognised  under  the  microscope;  and  even  the 
microscope  does  not  always  give  conclusive 
evidence  about  a  volcanic  glass. 

The  chemical  group  of  a  fine-grained  rock 
may  often  be  recognised  in  the  field  by  examining 
some  weathered  surface.  As  the  basic  rocks  are 
rich  in  iron,  their  weathered  surfaces  are  stained 
a  brown  rusty  colour;  whereas  an  acid  rock, 
owing  to  its  poverty  in  iron,  weathers  of  a  light 
or  grey  colour. 

A  more  precise  test  which  can  be  easily  applied 
depends  upon  the  different  weight  of  the  rocks. 
Quartz  is  a  comparatively  light  mineral,  weighing 
only  2.6  times  as  much  as  an  equal  volume  of 
water;  that  is  to  say,  its  specific  gravity  is  2.6. 
Hornblende  and  augite,  on  the  other  hand,  weigh 
3.2  times  as  much  as  an  equal  bulk  of  water; 
olivine  is  3.4  times,  and  magnetite  more  than 
5  times  heavier  than  an  equal  bulk  of  water. 
Hence,  as  an  acid  rock  is  rich  in  quartz  and 
contains  no  olivine  or  magnetite,  it  is  lighter  than 
a  basic  rock  rich  in  these  constituents.  The 
specific  gravity  of  a  rock  can  be  easily  determined 
by  weighing  a  specimen  in  air,  then  weighing  it 
suspended  by  a  fine  thread  in  water,  and  dividing 
the  weight  in  air  by  the  difference  between  the 
weights  in  air  and  in  water.  Thus — 

Weight  in  air 

*'          DTfference  between  weight  in  air  and  weight  in  water 

Specific  gravity  affords  a  test  of  the  amount 
of  silica  present  in  a  rock.  Thus,  according  to  a 
table  compiled  by  Dr.  Teall — 


SECONDARY  OR  STRATIFIED  ROCKS    37 

Average 

sp.  gr.  Silica 

23  varieties  of  granite          2.65    and  contain  71.6% 

10  ,,  syenite         2.82  „  63.8% 

13  „  gabbro          2.90  „  51 .5% 

5  ,,  peridotite    3.26  ,,  44-6% 

A  cubic  foot  of  average  granite    weighs  165  Ibs. 

.»       syenite          „     175     ,, 
,,  ,,  ,,       diorite  ,,      177    ,, 

,,  „  „       gabbro  ,,      180    „ 

>i  »  .,       peridotite      -,,     203    ,, 

This  simple  and  easily  applied  test  therefore  indi- 
cates the  approximate  amount  of  silica  present 
in  a  rock,  and  thus  enables  it  to  be  referred  to 
its  group  without  the  necessity  for  a  chemical 
analysis. 

CHAPTER  V 

THE   SECONDARY   OR   STRATIFIED    ROCKS 

THE  primary  materials  of  the  earth's  crust  consist 
of  an  igneous  material  which  has  been  given  off 
like  a  slag  from  the  consolidating  central  core, 
and  then  by  process  of  differentiation  has  divided 
into  two  types  of  slag;  the  acid  type  has  been 
aptly  called  the  salic,  from  the  initials  for  its 
two  chief  constituents,  silica  and  alumina;  the 
more  basic  product  is  the  femic  (see  p.  26).  The 
salic  product  constitutes  the  granites,  syenites, 
and  liparites — the  femic  supplies  the  gabbros, 
dolerites,  and  basalts;  various  mixtures  of  the 
two  form  the  intermediate  types,  such  as  diorite 
and  andesite. 

When  these  primary  rocks  are  exposed  to  the 
action  of  the  rain,  wind,  and  weather,  their 
constituents  are  gradually  decomposed.  Per- 
fectly pure  water  acting  at  a  constant  tempera- 


38  GEOLOGY 

ture  on  a  slab  of  fresh  granite  would  have 
practically  no  destructive  effect.  But  such  are 
not  the  conditions  met  with  in  nature.  Rain 
water  is  not  pure;  it  is  slightly  acid,  as  it  always 
contains  some  carbonic  acid  derived  from  the 
air,  as  well  as  some  oxygen.  The  rain  that  falls 
in  cities  generally  contains  sulphuric  and  hydro- 
chloric acids  as  well.  The  temperature,  moreover, 
varies  continually;  and  exposed  surfaces  of  rock 
expand  when  heated  and  contract  when  cooled; 
and  if  the  cooling  and  the  resulting  contraction 
be  rapid,  the  rock  is  torn  by  cracks.  Moreover, 
different  minerals  expand  at  different  rates  when 
warmed;  and  thus  a  rock  like  granite,  which  is 
composed  of  three  constituents,  is  rent  internally 
by  the  unequal  expansion  of  its  minerals.  The 
rain  water  soaks  into  the  cracks,  and  its  acids 
and  oxygen  are  thus  able  to  act  upon  the  minerals 
on  all  sides.  The  water  that  soaks  into  a  granite 
first  attacks  any  dark  -  coloured  mica  that  may 
be  present,  and  removes  its  iron;  the  carbonic 
acid  decomposes  the  felspar,  and  removes  in 
solution  its  lime,  potash,  or  soda.  The  rest  of 
the  felspar  remains  as  silicate  of  alumina,  or 
common  clay  substance,  mixed  with  any  excess 
of  silica  which  is  left  as  quartz.  The  original 
quartz  in  the  granite  is  not  appreciably  attacked, 
but  it  falls  out  after  the  destruction  of  the  felspar 
and  mica,  and  it  is  washed  away  by  the  rain. 

In  the  basic  igneous  rocks  analogous  changes 
happen:  the  oxygen,  owing  to  the  complex 
process  known  as  rusting,  combines  with  the  iron 
and  converts  it  into  an  iron  oxide  or  rust;  or  the 
carbonic  acid  may  combine  with  the  iron  to  form 
a  bicarbonate  of  iron,  which  is  removed  in  solution 
The  carbonic  acid  also  attacks  any  minerals  con- 


SECONDARY  OR  STRATIFIED  ROCKS    39 


taining  lime,  and  forms  a  bicarbonate  of  lime 
which  is  removed  in  solution.  Slowly  the  whole 
rock  decays  to  a  clay,  stained  brown  by  iron 
oxide.  The  fine  grains  of  clay  substance  are 
then  blown  away  by  the  wind  as  dust,  or  washed 
by  the  rain  into  streams  and  carried  away  as  silt. 
The  destruction  of  the  primary  rocks  gives 
rise,  then,  to  the  following  materials — 


Primary 
Rocks. 

Constituent 
Mineral 
Species. 

Chemical 
Composition. 

Method  of  Removal. 

Deposited  as 

Quartz 

vSilica 
(  Silicate     of 

Silica  in  suspension 
Silicate    of    alumina    in 

Sand  grains 
Clay 

alumina 

suspension 

Acid 

J  Silicate     of 

Carbonate  of  potash  in 

felspars 

1      potash 
Silicate     of 

solution 
Carbonate    of    soda    in 

soda 

solution 

i  Silicate     of  \ 

Granite  ^ 

White 
mica 

J      alumina    ( 
I  Silicate     of  f 

As    flakes    of    mica    in 
suspension 

Mica  flakes 

potash 

Brown 
mica 

r  Silicate     of  \ 
1       alumina    / 
Silicate     of  f 

As  flakes  of  secondary 
decomposition     pro- 
ducts 

(if  present) 

1      magnesia  ' 
Silicate     of 

Iron   removed   in   solu- 

I 

>»     iron 

tion 

( 

r  Silicate     of 

Clay  in  suspension 

Clay 

alumina 

Basic 

Silicate     of  \ 

Secondary  silica  in  sus- 

Sand grains 

1 

felspars 

1     .lime           ( 

pension 

! 

I  Silicate     of  I 

Bicarbonates  of  lime  and 

Lime   in 

soda 

soda  in  solution 

limestone  ; 

soda  in  salt 

Gabbro^J 

deposits 

1 

(  Silicate     of 

Clay  in  suspension 

Clay 

alumina 

Ferro- 

Silicate     oK 

Secondary  silica  in  sus- 

Sand   grains 

magne- 

magnesia 

pension. 

sian 

Silicate     of  I 

)  Bicarbonates   of    lime, 

Ljmestone, 

mineral 

iron 
Silicate     of 

magnesia,    and    iron 
in  solution 

etc. 

I 

lime 

40  GEOLOGY 

The  new  materials  are  distributed  by  wind  and 
water,  and  laid  down  as  stratified  deposits.  These 
beds  may  be  cemented  and  form  secondary  rocks, 
which  are  classified  according  to  material  or  mode 
of  origin.  Most  secondary  rocks  are  formed 
from  fragments  of  primary  rocks,  laid  down  as 
sediments;  hence  secondary  rocks  are  sometimes 
called  sedimentary  rocks,  and  sometimes  they 
are  called  clastic  rocks,  from  the  Greek  word 
klastos,  broken. 

If  these  rocks  are  formed  by  the  agency  of  the 
wind,  they  are  called  aeolian  or  subaerial  rocks; 
if  formed  by  water,  they  are  called  aqueous  rocks ; 
the  deposits  due  to  ice  are  described  as  glacial. 

The  materials  of  some  secondary  rocks  are 
carried  in  solution  by  water,  and  their  solution 
and  redeposition  are  due  to  chemical  or  organic 
and  not  to  mechanical  processes.  The  materials 
may  be  extracted  from  the  water,  either  by  the 
action  of  some  animal  or  plant  which  has  the 
power  of  extracting  the  material  from  solution 
and  secreting  it  as  its  shell,  or  skeleton,  or  stem. 
These  rocks  are  known  as  the  organically  formed 
rocks.  The  material  may,  however,  be  removed 
from  the  water  by  some  chemical  process,  and 
such  deposits  are  said  to  be  chemically  formed. 

The  material  therefore  obtained  by  the  de- 
struction of  the  primary  rocks  is  redeposited  as 
secondary  rocks  by  three  processes,  which  give 
rise  to  three  groups  of  rocks — the  sedimentarily, 
the  organically,  and  the  chemically  formed. 


THE  SEDIMENTARY  DEPOSITS        41 


CHAPTER  VI 

THE   SEDIMENTARY   DEPOSITS 

THE  rocks  formed  by  the  mechanical  transport  and 
deposition  of  material  derived  from  the  primary 
rocks  are  most  conveniently  classified  according  to 
the  nature  of  their  chief  constituent.  There  are 
two  main  groups  of  sediments — the  arenaceous 
(from  Latin  arena,  a  sand  grain)  and  the  argil- 
laceous (from  Latin  argilla,  clay). 

The  Arenaceous  Series. — The  simplest  member 
of  the  arenaceous  series  is  sand,  which  consists 
of  small  grains  of  various  minerals ;  the  commonest 
is  quartz,  as  owing  to  its  hardness  and  durability, 
its  reduction  to  very  fine  powder  is  a  very  slow 
process.  The  sands  found  in  the  British  Islands 
are  usually  composed  of  siliceous  fragments; 
hence  sand  is  often  spoken  of  as  if  it  were  always 
siliceous.  But  in  many  localities  the  sands  are 
composed  of  grains  of  felspars;  many  sands  on 
the  sea-shore  consist  of  carbonate  of  lime  formed 
by  the  breaking  up  of  shells  or  corals.  The  white 
sands  of  the  beaches  on  the  Pacific  coral  islands 
are  composed  of  grains  of  corals  and  shells,  and 
so  are  calcareous.  Sand  grains  may  be  rough, 
sharp,  and  angular,  as  in  ordinary  sea  sand; 
but  those  that  have  been  rolled  about  by  the 
wind,  as  on  a  desert  or  a  sand  dune,  may  be  as 
well  polished  and  as  perfectly  rounded  as  are 
a  boy's  marbles. 

The  two  essential  qualities  of  sand  are  that 
the  grains  must  be  loose,  and  that  they  must 
be  more  than  .05  of  a  millimetre,  or  more  than 
one-five-thousandth  of  an  inch  in  diameter.  If 


42  GEOLOGY ' 

the  grains  be  smaller,  the  material  is  a  clay.  If 
the  grains  be  fastened  together  by  some  cement 
into  a  firm  coherent  rock,  then  the  material  is 
changed  from  sand  into  sandstone.  If  the  cement 
be  so  firm  that  the  rock  breaks  with  a  smooth 
instead  of  a  rough  surface,  the  fracture  passing 
through  the  sand  grains  as  easily  as  around  them, 
the  rock,  if  siliceous  in  composition,  is  called  a 
quartzite. 

In  an  ordinary  sandstone  the  grains  are 
small.  If  the  grains  are  larger,  the  rock  is  known 
as  a  grit,  like  the  millstone  grit.  If  they  are  as 
large  as  pebbles,  the  rock  is  either  a  conglomerate, 
if  the  pebbles  be  rounded  and  angular,  or  a  breccia 
if  they  be  angular.  Conglomerate  and  breccia 
are  cemented  gravel  or  pebble  beds,  just  as  sand- 
stone is  cemented  sand. 

The  Argillaceous  Series. — The  typical  member 
of  the  argillaceous  series  is  clay,  and  the  essential 
feature  of  a  clay  is  that  it  is  so  fine  grained 
that  when  moistened  it  is  soft  and  plastic.  This 
property  belongs  to  most  materials  of  which 
the  grains  are  less  than  one-five-thousandth  of 
an  inch  in  diameter.  Most  clays,  however,  are 
formed  of  fine  particles  of  decomposed  felspar, 
and  they  consist  chemically  of  a  silicate  of  alumina 
combined  with  water.  But  clay  substance  may 
consist  of  other  materials,  such  as  quartz  ground 
so  finely  that  when  rubbed  between  the  fingers 
it  feels  soft  like  flour. 

Most  clay  is  deposited  by  water  on  the  floor 
of  seas  or  lakes,  or  in  the  quiet  parts  of  river 
channels;  it  is  then  usually  saturated  with  water, 
and  contains  much  organic  matter,  and  is  known 
as  mud. 

A  clay  so  compact  that  it  cannot  be  broken 


CHEMICALLY  FORMED  ROCKS        43 

in  the  hand  and  is  not  plastic,  until  after  it  has 
been  ground  to  powder,  is  a  mudstone  or  clay- 
stone. 

Clay  is  deposited  under  water  in  successive 
layers,  and  when  the  clay  is  dried  and  slightly 
compressed,  it  generally  breaks  readily  along 
the  planes  between  these  layers;  this  variety  of 
clay  is  known  as  shale. 

If  clay  be  subjected  to  heavy  pressure,  the 
constituents  are  rearranged  so  that  all  the  par- 
ticles lie  with  their  larger  surfaces  parallel,  and 
at  right  angles  to  the  direction  of  the  pressure. 
The  rock  may  then  split  readily  into  thin  smooth 
slabs;  rock  in  this  condition  is  known  as  slate, 
and  the  property  by  which  it  thus  divides  is 
its  slaty  cleavage. 

A  mixture  of  sand  and  clay  is  a  loam;  and  a 
mixture  of  clay  with  calcareous  material  is  known 
as  marl.  If  the  sand,  sandstone,  or  clay  contain 
abundant  flakes  of  mica,  it  is  then  said  to  be 
micaceous;  if  it  contain  much  iron,  it  is  a 
ferruginous  rock  or  ironstone. 


CHAPTER  VII 

CHEMICALLY   FORMED   ROCKS 

IN  addition  to  the  stratified  rocks  that  are  formed 
directly  of  redeposited  fragments  of  igneous 
rocks,  there  are  two  kinds  of  rocks  which  are 
secondary  in  the  sense  that  their  materials  are 
derived  from  the  primary  rocks;  but  this  origin 
is  less  obvious,  as  the  material  has  been  removed 
from  the  igneous  rocks  in  solution,  and  ex- 
tracted from  the  water  that  dissolved  it  by  some 


44  GEOLOGY 

chemical  reaction  or  by  living  beings.  The  rocks 
are  therefore  known  as  either  chemically  deposited 
or  organically  deposited. 

The  chemically  deposited  rocks  are  compara- 
tively small  in  quantity,  but  they  are  important 
owing  to  their  high  economic  value.  They  may 
be  classified  according  to  either  their  composition 
or  mode  of  formation.  Under  the  first  system 
these  rocks  are  divided  into  the  calcareous — those 
formed  of  carbonate  of  lime;  the  siliceous — 
those  formed  of  silica;  the  ferruginous — those 
rich  in  iron;  and  the  carbonaceous — in  which 
the  most  important  element  is  carbon.  Under 
the  second  and  more  convenient  system  there 
are  rocks  formed  (i)  by  chemical  reactions;  (2) 
by  the  evaporation  of  the  water  from  lakes  or 
lagoons;  (3)  as  an  efflorescent  crust  on  the  land 
by  the  evaporation  of  solutions  rising  from  under- 
ground. 

i.  Chemically  deposited  Carbonates. — The  most 
widely  distributed  rock  that  has  been  formed  chemi- 
cally is  composed  of  carbonate  of  lime.  Its  forma- 
tion is  so  important  that  it  is  advisable  to  follow 
the  process  carefully.  The  lime  present  in 
igneous  rocks  is  usually  present  as  a  silicate  of 
lime;  water  containing  some  dissolved  carbonic 
acid  (COa)1  which  it  has  obtained  from  the  air 
acts  upon  the  mineral,  decomposes  the  silicate 
of  lime,  and  removes  the  lime  as  bicarbonate  of 
lime  (composed  of  one  molecule  of  lime,  CaO,  com- 
bined with  two  molecules  of  CO2),  which  is  soluble 
in  water. 

1  CO2,  carbonic  dioxide,  in  accordance  with  the  usual 
practice  of  geologists,  is  here  called  carbonic  acid;  but  it 
is  not,  strictly  speaking,  an  acid  until  combined  with  water. 


CHEMICALLY  FORMED  ROCKS         45 

The  reactions  are  shown  as  follows — 

CaO,SiO.,  +       2C02      +      H.,0       =    SiO,,1      CaO,H2O,2CO22 
(Silicate"        (Carbonic        (Water)         (Silica)      (Bicarbonate 
of  lime)  acid)  of  lime) 

An  analogous  process  happens  when  water 
containing  carbonic  acid  in  solution  acts  upon 
limestone,  which  is  composed  of  one  molecule 
of  lime  (CaO)  combined  with  one  molecule  of 
carbonic  acid  or  carbon  dioxide  (CO2).  Its  for- 


Fig.  5. 

A  Cave  formed  by  solution  of  Limestone  (L).  A  and  B  are 
layers  impermeable  to  water,  c,  stalactites  on  the  roof  of  the 
cave,  beneath  which  are  the  humps  of  stalagmite  on  the  floor. 
M,  mouth  of  the  cave. 

mula,  therefore,  may  be  written  either  CaCO3  or 
CaO,CO2.  As  it  contains  only  one  part  of  car- 
bonic acid  united  with  one  part  of  lime,  it  is  a  uni- 
carbonate;  and  unicarbonate  of  lime  is  insoluble 
in  water.  If  water  containing  carbonic  acid 
passes  into  a  bed  of  limestone,  the  carbonic  acid 
unites  with  some  of  the  unicarbonate  of  lime 
and  converts  it  into  the  soluble  bicarbonate  of 
lime  which  is  removed  in  solution.  If  this  water 
be  subsequently  exposed  to  the  air,  the  extra 
molecule  of  carbonic  acid  may  escape,  with  the 
result  that  the  lime  compound  is  altered  to  the 

1  Left  behind  as  particles  of  quartz. 

2  Removed  in  solution. 


46  GEOLOGY 

insoluble  unicarbonate,  and  deposited  as  a  layer 
of  carbonate  of  lime.  Thus,  when  water,  after 
percolating  through  limestone,  reaches  a  cave, 
the  drops  of  water  hang  upon  the  roof;  the  extra 
molecule  of  carbonic  acid  is  given  off,  and  the 
carbonate  of  lime  deposited.  More  such  water 
oozing  from  the  same  place  deposits  more  car- 
bonate of  lime,  and  gradually  forms  from  the 
roof  of  the  cave  a  pendent,  called  a  stalactite. 
The  beauty  of  caves  in  limestone  districts  is  due 
to  the  fantastic  forms  and  translucency  of  these 
stalactites.  The  water  from  the  tip  of  the  stalac- 
tite falls  upon  the  floor,  and  there  deposits  the 
rest  of  its  carbonate  of  lime  in  a  thin  film.  Suc- 
cessive films  form  a  compact  sheet  or  dome- 
shaped  humps  of  limestone,  which  is  known  as 
stalagmite  (Fig.  5). 

When  a  spring  issues  from  a  calcareous  rock, 
there  also  the  water  may  deposit  carbonate  of 
lime,  as  in  a  slight  crust  on  any  substance  over 
which  the  water  spreads  in  a  thin  layer,  thus  ex- 
posing a  wide  surface  to  the  air.  Such  springs  are 
popularly  known  as  petrifying  springs.  The  car- 
bonate of  lime  deposited  by  spring  water  around 
twigs  and  mosses  sometimes  accumulates  as  beds 
of  porous  limestone,  known  as  calcareous  tufa. 
The  precipitated  carbonate  of  lime  may,  however, 
be  carried  away  in  fine  particles  by  the  stream 
from  the  spring,  and  deposited  in  some  quiet  pool 
or  lake  as  a  bed  of  chemically  formed  limestone. 

Other  materials,  as  well  as  lime,  have  a  soluble 
bicarbonate  and  an  insoluble  unicarbonate,  and 
chemical  deposits  of  these  materials  are  formed 
by  analogous  processes.  The  most  important 
are  some  deposits  of  carbonate  of  iron. 

Another  kind  of  chemically  formed  rock  is  due 


CHEMICALLY  FORMED  ROCKS         47 

to  water  containing  a  solvent  percolating  through 
a  rock  and  dissolving  one  of  its  constituents.  The 
dissolved  matter  may  be  deposited  as  nodules  or 
masses  in  spaces  existing  in  the  rock ;  the  spaces 
may  be  formed  by  particles  of  a  rock  being  dissolved 
and  replaced,  almost  at  once,  by  fresh  material. 
Thus  water  containing  alkali  will  dissolve  silica, 
which  it  may  .redeposit  as  lumps  or  nodules  of 
"  chert."  Carbonate  of  magnesia  may  be  carried 
into  limestones,  and  there,  combining  with  some  of 
the  carbonate  of  lime,  converts  the  limestones  into 
the  rock  known  as  dolomite. 

2.  Ore  Deposits. — The  solution  •  of  particles 
widely  scattered  through  rocks  and  their  collec- 
tion or  "  segregation,"  either  in  a  fissure  or  in 
large  masses,  is  the  process  to  which  we  owe 
the  majority  of  ore  deposits.  Most  metal-bearing 
veins  or  lodes  are  due  to  hot  solutions  that  have 
arisen  from  a  great  depth  below  the  earth's 
surface,  where  they  dissolved  particles  of  various 
metals.  As  the  solutions  approached  the  surface 
they  were  cooled,  and  the  minerals  dissolved  in 
them  deposited  along  their  channels  as  mineral 
lodes  or  veins. 

Some  ore  deposits,  especially  iron  ores,  are 
formed  by  water  soaking  downward  from  the  sur- 
face and  depositing  iron  collected  from  the  over- 
lying rocks. 

Mineral  lodes  formed  near  the  surface  of  the 
earth  generally  contain  carbonates,  such  as  car- 
bonate of  lime.  The  deeper  ore  deposits  generally 
contain  much  quartz,  mixed  with  various  metals 
or  metallic  compounds  scattered  through  it. 

Many  of  the  larger  ore  deposits  occur  as  vast 
masses  of  an  irregular  egg-shaped  or  lens-shaped 


48  GEOLOGY 

form.  They  cannot  have  been  formed  in  cavities, 
as  no  spaces  so  large  could  have  remained  open 
deep  within  the  earth.  They  are  due  to  the  re- 
placement process.  Solutions  have  removed  the 
original  rock,  particle  by  particle,  replacing  each 
at  the  same  time  by  ore. 

3.  Deposits  formed  by  Evaporation. — A  third 
series  of  chemically  formed  rocks  are  those  pro- 
duced by  the  evaporation  of  sheets  of  water, 
such  as  lakes,  or  former  arms  of  the  sea  which 
have  been  completely  enclosed  by  land.  Rivers 
are  constantly  adding  mineral  matter  to  the  sea, 
and  to  lakes  which  have  no  outlets.  This 
material  accumulates  until  it  amounts  to  an 
average  in  the  water  of  the  existing  seas  of  about 
3j  parts  of  mineral  matter  in  every  100  parts 
of  water.  This  3^  per  cent,  is  composed  of  the 
following  constituents — 


Salt  (sodium  chloride) 
Chloride  of  magnesium 
Sulphate  of  magnesium 
,,  lime 

, ,  potash    . 

Bromide  of  magnesium 
Carbonate  of  lime 


77-8 
10.9 

4-7 
3-6 

2.5 

.2 

•3 


IOO.O 


During  the  evaporation  of  sea  water  the  least 
soluble  of  its  salts  are  deposited  first,  and  the 
most  soluble  last.  Sulphate  of  lime  is  much  less 
readily  dissolved  in  water  than  common  salt. 
Hence  as  the  water  evaporates  the  sulphate  of 
lime  is  deposited  first,  and  forms  beds  of  gypsum. 
Later  on,  if  all  or  nearly  all  the  water  be  removed, 
chloride  of  sodium  is  deposited  as  common  salt. 

Rock  salt  is  the  chief  source  of  the  common  salt 


CHEMICALLY  FORMED  ROCKS    49 

(sodium  chloride)  used  in  the  British  Isles;  it  is 
obtained  from  beds  in  Cheshire  and  Worcester- 
shire on  the  sites  of  old  lagoons.  Sulphate  of  lime 
combined  with  water  forms  gypsum,  commercially 
valuable  since  plaster  of  Paris  is  prepared  from  it. 
The  massive  varieties,  some  of  which  are  streaked 
with  pink  veins  due  to  colouring  by  oxide  of  iron, 
are  known  as  alabaster  and  are  worked  as  an 
ornamental  stone.  Alabaster  is  easily  carved,  being 
so  soft  that  it  can  be  scratched  with  the  thumb-nail. 
The  evaporation  of  lakes  lays  down  beds  of 
gypsum  and  salt,  and  of  other  materials,  such  as 
carnallite  (chloride  of  potassium  and  magnesium) 
and  natron  (carbonate  of  soda). 

4.  Efflorescent  Rocks  comprise  a  fourth  series 
of  chemically  formed  rocks.  They  are  due  to 
solutions  containing  various  materials  being 
sucked  up  to  the  surface  of  the  earth  by  capillary 
attraction  and  there  evaporated.  They  may  then 
form  a  thin  crust  of  limestone,  carbonate  of  soda 
(thermonatrite) ,  ironstone,  or  chert.  Efflorescent 
rocks  are  not  common  in  countries  with  a  wet 
climate,  for  they  would  be  washed  away  as  quickly 
as  they  were  formed;  but  in  arid  regions,  where 
evaporation  would  remove  more  water  than  falls 
as  rain,  these  efflorescent  rocks  are  very  wide- 
spread. In  some  places  they  enrich  the  soil  by 
bringing  up  plant  food  from  below;  but  in  other 
cases  they  are  fatal  to  agriculture  by  covering  the 
surface  with  a  layer  of  hard  chert,  or  saturating 
the  soil  with  some  injurious  constituent. 


50  GEOLOGY 


CHAPTER  VIII 

ORGANICALLY   FORMED    ROCKS 

THESE  rocks  are  due  to  the  action  of  animals 
and  plants  which  extract  various  materials  from 
water,  soil,  or  air,  and  fix  them,  in  their  skeletons, 
shells,  or  hard  tissues.  After  the  death  of  the 
animals  or  plants  their  hard  parts  may  endure,  and 
if  buried  and  thus  preserved  they  are  known  as 
fossils.  Fossil  remains  may  accumulate  in  suffi- 
cient quantities  to  form  entire  beds  of  rock  of 
great  thickness.  Some  organically  formed  lime- 
stones are  thousands  of  feet  thick,  and  extend  over 
many  thousand  square  miles.  Such  vast  sheets 
of  limestone  cannot  .have  been  deposited  chemi- 
cally from  sea  water,  as  it  does  not  contain 
enough  carbonate  of  lime.  Sea  water  contains, 
on  an  average,  3.6  parts  in  100  of  sulphate  of  lime, 
and  only  one-eleventh  of  that  quantity  of  car- 
bonate of  lime.  Animals  that  live  in  the  sea 
and  have  shells  of  carbonate  of  lime  obtain  their 
lime  from  the  sulphate ;  for  they  have  been  observed 
by  Sir  John  Murray  to  live  and  form  their  shells 
in  water,  from  which  all  the  carbonate  of  lime 
has  been  artificially  removed.  They  convert  the 
sulphate  of  lime  into  carbonate  of  lime  by  a 
chemical  reaction — 

CaSO4      +       Am2CO3      =      CaCO3      +      Am2SO4 
(Sulphate          (Ammonium         (Carbonate         (Ammonium 
of  lime)     .          carbonate)  of  lime)  sulphate) 

produces  ammonium  sulphate  and  carbonate  of  lime — 

that  takes  place  in  their  bodies. 

Many   different   kinds   of   animals    and    plants 
form  hard  skeletons,  which   after  death   remain 


ORGANICALLY  FORMED  ROCKS       51 

as  the  constituents  of  organically  formed  rocks. 
Some  animals  form  skeletons  of  carbonate  of 
lime  and  give  rise  to  calcareous  rocks;  others, 
as,  for  example,  some  sponges,  have  their  hard 
parts  of  silica,  and  form  siliceous  rocks;  many 
plants  have  tissues  of  carbonaceous  material,  and 
give  rise  to  beds  of  peat  and  coal. 

Organically  formed  Calcareous  Rocks. — The  most 
numerous  animals  that  have  skeletons  of  carbonate 
of  lime  belong  to  the  group  of  the  Mollusca,  which 
includes  the  ordinary  shell-fish,  such  as  the  oyster, 
cockle,  mussel,  whelk,  and  the  land  snails.  Mol- 
lusca are  abundant  in  the  sea,  in  lakes,  in  rivers, 
and  on  land.  They  often  live  in  colonies,  such 
as  oyster  beds,  and  on  the  death  of  the  animals 
their  hard  shells  accumulate  as  shell  beds.  The 
spaces  between  the  shells  may  be  filled  by  shell 
fragments,  or  all  the  shells  may  be  broken  into 
small  particles  forming  shell  sand.  Water  sub- 
sequently percolating  through  the  mass  may 
cement  it  into  a  firm  rock. 

Corals  that  live  in  shallow  water  in  tropical 
seas  have  thick  skeletons  of  carbonate  of  lime, 
and  as  they  grow  in  colonies,  they  form  masses 
of  coral  limestone  known  as  coral  reefs.  These 
reefs  sometimes  form  islands,  and  sometimes  long 
breakwaters  skirting  the  shore. 

Sea  lilies  (Crinoids)  are  a  group  of  animals 
allied  to  the  starfish.  Most  of  them  live  attached 
to  the  sea  floor  by  a  long  flexible  stem  composed 
of  many  short  joints  of  dense  carbonate  of  lime. 
The  stems  are  sometimes  100  feet  in  length,  and 
on  the  death  of  the  sea  lily  the  joints  of  the  stem 
fall  apart  and  are  scattered  over  the  sea  floor. 
They  are  called  St.  Cuthbert's  beads,  and  form 
beds  of  limestone  known  as  crinoid  limestone,  of 


52  GEOLOGY 

which  some  kinds,  such  as  the  Derbyshire  marble, 
are  worked  as  ornamental  stones. 

The  Bryozoa,  so  named  from  their  moss-like 
appearance,  are  small  compound  animals;  they 
often  grow  in  dense  clusters,  and  their  small  stem 
fragments  form  bryozoal  limestones.  The  Fora- 
minifera  are  minute  animals  of  a  very  simple 
organisation;  most  of  them  have  a  shell  of  car- 
bonate of  lime;  they  live  in  great  abundance  on 
the  sea  floor  or  on  the  sea  surface.  Their  dead 
shells  accumulate  to  form  a  widespread  sheet  of 
calcareous  earth  known  as  foraminiferal  ooze, 
which  covers  vast  tracts  of  the  ocean  bed.  These 
foraminiferal  deposits  may  be  consolidated  into 
limestones,  and  some  of  the  most  important  and 
best -known  limestones  are  principally  composed 
of  foraminiferal  shells.  The  Pyramids  are  mainly 
built  of  such  limestone,  but  the  Foraminifera  in 
them  are  of  a  comparatively  gigantic  kind,  form- 
ing thin  discs  about  as  large  as  a  penny. 

Plants  also  help  in  the  formation  of  limestones. 
The  oolitic  limestones,  which  are  largely  used  in 
England  for  building-stone,  are  composed  of  small 
rounded  grains,  so  that  the  rock  resembles  the 
hard  roe  of  a  fish.  These  grains  were  at  first 
thought  to  have  been  formed  chemically  by  the 
repeated  evaporation  of  sea  water  that  had 
moistened  beds  of  sand,  a  thin  film  of  carbonate 
of  lime  being  deposited  on  each  occasion.  This 
process  is  chemically  improbable  owing  to  the 
scarcity  of  carbonate  of  lime  in  sea  water;  and 
the  microscopic  study  of  these  oolitic  grains 
shows  that  they  have  been  formed  by  seaweeds 
or  algae,  which  extract  lime  from  sea  water  and 
deposit  it  as  carbonate. 

Organically    formed    Siliceous    Rocks.  —  Organi- 


ORGANICALLY  FORMED  ROCKS       53 

cally  formed  siliceous  rocks  are  due  to  the  action 
of  organisms  which  build  their  skeletons  of  silica. 
Many  sponges  have  skeletons  composed  of  minute 
rods  or  spicules  of  silica.  The  spicules  left  after 
the  death  of  the  sponges  on  a  sponge  bank 
may  accumulate  and  be  cemented  into  chert. 
Radiolaria  are  primitive  animals  closely  related 
to  the  Foraminifera,  from  the  typical  kinds  of 
which  they  differ  by  having  shells  composed  of 
silica.  The  existing  Radiolaria  mostly  live  far 
from  land  in  the  tropical  oceans ;  their  dead  shells 
form  deposits  of  radiolarial  ooze  widely  spread  over 
the  ocean  floor.  Some  kinds  of  chert  are  radiolarian 
ooze  cemented  into  a  hard  rock. 

Many  lakes,  as  well  as  the  colder  seas,  are 
inhabited  by  vast  swarms  of  minute  plants,  known 
as  diatoms,  which  have  a  shell  composed  of  silica. 
Their  shells  accumulate  on  the  floors  of  the 
oceans  as  a  diatomaceous  ooze,  and  in  lakes  they 
often  form  thick  deposits  of  almost  pure  siliceous 
powder.  This  material  is  used  for  polishing  stone 
and  metal  under  the  name  of  Tripoli  Powder. 

The  plants  known  as  Algae  extract  silica  from 
the  waters  of  hot  springs,  and  deposit  it  in  crater- 
shaped  mounds  or  terraces,  which  are  often  of 
exquisite  beauty.  This  variety  of  siliceous  rock 
is  known  as  sinter.  The  "  Pink  and  White 
Terraces  "  of  New  Zealand  are  the  most  famous 
and  were  probably  the  most  beautiful  of  these 
sinter  formations;  but  they  were  destroyed  by 
a  volcanic  explosion  which  blew  them  into  frag- 
ments during  the  eruption  of  Mount  Tarawera  in 
1886. 

Phosphatic  Deposits. — Deposits  of  phosphate  of 
lime  are  much  scarcer  than  those  of  carbonate 
of  lime,  but  they  are  of  great  economic  value  as 


54  GEOLOGY 

one  of  the  chief  sources  of  artificial  manures. 
One  of  the  best  known  is  guano,  which  is  com- 
posed of  the  droppings  of  sea  birds  on  small 
islands  in  arid  regions.  Fish  are  rich  in  phos- 
phorus, and  contain  more  than  the  birds  which 
prey  upon  them  can  assimilate;  the  rest  passes 
from  the  birds  in  their  dung.  The  droppings 
fall  mainly  around  the  nests  and  breeding-places. 
The  birds  cannot  assemble  for  breeding  in  large 
numbers  on  the  mainland,  or  they  would  be  preyed 
upon  by  animals  and  their  eggs  devoured  by 
snakes.  The  chief  breeding-places  of  sea  birds, 
therefore,  are  usually  on  small  islands. 

Guano,  moreover,  can  only  form  where  there 
is  a  small  rainfall,  as  otherwise  its  valuable  con- 
stituents would  be  washed  out  and  carried  to 
the  sea;  it  is  therefore  a  rare  deposit,  because 
it  can  only  accumulate  under  exceptional  geo- 
graphical conditions.  The  chief  supplies  have 
come  from  the  Guano  Islands  off  the  western 
coast  of  South  America,  in  the  Central  Pacific, 
from  islands  off  south-western  Africa,  off  the 
Australian  coast,  and  in  the  West  Indies. 

If  a  bed  of  bird-droppings  be  soaked  by  rain,  the 
water  dissolves  the  soluble  constituents,  including 
the  phosphoric  acid,  and  carries  them  down  into 
the  underlying  rocks;  there  the  phosphoric  acid 
reacts  with  one  of  the  constituents  of  the  rock, 
and  forms  a  phosphate  rock.  If  the  island  be 
of  coral  limestone,  the  carbonate  of  lime  is  altered 
into  phosphate  of  lime;  if  it  be  a  volcanic  island, 
some  of  the  earthy  minerals  are  converted  into 
aluminium  phosphate.  Guano,  as  distinguished 
from  rock  phosphate,  can  be  used  at  once  as  a 
manure ;  but  the  rock  phosphates  require  chemical 
treatment,  so  that  their  phosphoric  acid  may  be 


ORGANICALLY  FORMED  ROCKS       55 

rendered  soluble  by  water,  and  thus  made  available 
as  a  food  for  crops. 

Some,  phosphatic  deposits  are  formed  by 
accumulations  of  bat  dung  in  caves,  or  by  the 
accumulation  of  bones,  which  are  composed 
mainly  of  phosphate  of  lime.  Bones  often  collect 
in  swamps  in  which  animals  are  bogged  during 
their  efforts  to  get  to  water,  and  in  lakes  into 
which  dead  animals  are  washed  by  floods.  Copro- 
lites,  the  fossil  dung  of  large  land  animals,  are 
also  a  source  of  phosphate. 

Many  igneous  rocks  contain  small  crystals  of 
apatite,  a  mineral  composed  mainly  of  phosphate 
of  lime.  These  crystals  are  dissolved  and  their 
material  carried  in  solution  to  the  sea,  where  the 
phosphoric  acid  may  act  upon  minute  calcareous 
organisms  and  convert  them  into  .grains  of  phos- 
phate of  lime.  The  accumulation  of  these  grains 
may  form  beds  of  phosphatic  limestone  or  phos- 
phatic chalk. 

Carbonaceous  Rocks. — The  last  group  of  organi- 
cally formed  rocks  includes  those  formed  from  plant 
remains;  they  are  known  as  carbonaceous  rocks, 
as  carbon  is  their  chief  constituent.  They  are  in- 
valuable as  our  chief  source  of  fuel.  All  vegetation 
contains  a  considerable  proportion  of  the  element 
carbon.  Where  leaves  and  branches  of  trees 
accumulate  on  the  floor  of  a  forest,  they  form  a 
leaf  mould  rich  in  carbon.  The  growth  of  mosses 
in  swamps  forms  a  spongy,  sodden  mass,  which 
dries  into  the  material  known  as  peat.  If  forest 
mould,  or  a  bed  of  peat,  be  buried  beneath  a  layer 
of  sand  or  clay,  the  carbonaceous  layer  would  be 
preserved  and  represent  the  first  stage  in  the 
formation  of  a  coal  seam.  Wood  dried  in  the 
air  contains  about  20  per  cent,  of  water,  and  the 


56  GEOLOGY 

rest  consists  of  I  per  cent,  of  ash,  39  per  cent,  of 
carbon,  4j  per  cent,  of  hydrogen,  and  35 J  per  cent, 
of  oxygen.  Air-dried  peat  contains  on  an, average 
16  per  cent,  of  moisture,  8J  per  cent,  of  ash, 
44^  per  cent,  of  carbon,  4^  per  cent,  of  hydrogen, 
and  26J  per  cent,  of  oxygen  (including  a  little 
nitrogen).  If  either  leaf  mould  or  peat  be  sub- 
jected to  combined  heat  and  pressure,  the  volatile 
constituents  pass  away,  so  that  the  material  is 
left  with  a  higher  percentage  of  carbon.  Soft, 
spongy  peat  may  be  compressed  into  the  harder 
and  more  compact  fuel  known  as  lignite  or  brown 
coal,  which  contains,  on  an  average,  about  15 
per  cent,  of  moisture,  10  per  cent,  of  ash,  45  per 
cent,  of  carbon,  3f  per  cent,  of  hydrogen,  and 
26J  per  cent,  of  oxygen.  Increased  pressure 
may  convert  the  brown  coal  into  ordinary  black 
coal,  or,  as  it  is  often  called,  bituminous  coal; 
it  contains,  on  an  average,  3  per  cent,  of  moisture, 
10  per  cent,  of  ash,  72  per  cent,  of  carbon,  4  per 
cent,  of  hydrogen,  and  n  per  cent,  of  oxygen.  If 
still  more  of  the  volatile  constituents  be  removed, 
leaving  a  material  with  3  per  cent,  of  ash,  2  per  cent, 
of  moisture,  91 J  per  cent,  of  carbon,  2  J  per  cent,  of 
hydrogen,  and  I  per  cent,  of  oxygen,  the  coal 
has  been  converted  into  anthracite  or  smokeless 
coal.  This  is  much  harder  than  ordinary  coal; 
it  has  a  bright  lustre,  and  when  free  from  dust 
does  not  soil  the  hands.  It  is  not  readily  ignited, 
but  when  once  lighted,  burns  without  smoke  or 
flame  and  gives  a  more  intense  heat  than  ordinary 
bituminous  coal.  It  is  often  known  as  steam 
coal,  and  being  smokeless  is  especially  adapted 
for  naval  purposes,  as  the  presence  of  a  fleet 
using  it  would  not  be  betrayed  by  smoke.  If 
bituminous  coal  or  anthracite  be  subjected  to 


ORGANICALLY  FORMED  ROCKS       57 


intense  heat,  as  by  contact  with  a  dyke  of  igneous 
rock,  the  coal  is  converted  into  coke  or  graphite, 
which  consists  of  almost  pure  carbon ;  the  whole 
of  the  volatile  constituents  have  been  driven  off, 
just  as  coke  is  formed  in  a  gas-works  by  driving  off 
the  gas  from  coal. 

It  is  not  certain  that  the  difference  between 
bituminous  coal  and  anthracite  is  always  due  to 
the  latter  having  been  subjected  to  greater  heat 
and  pressure,  for  both  materials  occur  together, 
where  they  must  have  been  subject  to  the  same 
influences.  In  these  cases  the  two  kinds  of  coal 
have  probably  been  formed  from  different  kinds 
of  vegetation. 

The  gradual  transition  from  vegetable  matter 
to  graphite  is  shown  in  the  following  table,  which 
summarises  the  figures  given  in  the  previous 
paragraphs — 


Carbon. 

Hydro- 
gen. 

Oxygen. 

Ash. 

Mois- 
ture. 

Air  -dried  wood   . 

per  cent 
39 

per  cent 
4-5 

per  cent 
35-5 

per  cent 
i 

percent 

20 

Air  -dried  peat     . 

44-5 

4-5 

26.5  1 

8-5 

16 

Air-dried    brown 

coal 

45 

3-75 

26.25 

10  2 

15 

Bituminous    or 

black  coal 

72 

4.0 

I  I.O 

10  2 

3 

Anthracite     . 

91.5 

2-5 

I.O 

3 

2 

Graphite  . 

95 

— 

5 

— 

Oil  shale  and  cannel  coal  are  two  varieties  of 
coal  much  richer  in  oils  than  bituminous  coal. 
When  heated  they  give  off  the  hydrocarbons,  such 

1  Includes  a  little  nitrogen.  z  Very  variable. 


58  GEOLOGY 

as  mineral  paraffin,  which  consists  of  86  per  cent, 
of  carbon  united  with  14  per  cent,  of  hydrogen. 
Some  hydrocarbons  are  found  in  fissures  and  veins, 
and  they  are  probably  due  to  distillation  from 
carbonaceous  rocks.  The  richness  of  oil  shales 
in  hydrocarbons  is  probably  due  to  the  abundance 
in  them  of  spores  or  small  water  weeds,  instead  of 
the  woody  tissues  of  plants  and  trees  that  form 
ordinary  coal. 

The  oils,  such  as  petroleum,  which  spout  to  the 
surface  of  the  earth  in  many  regions,  such  as 
Baku  on  the  Caspian  and  the  oil  fields  of  America, 
have  probably  been  distilled  from  underlying 
carbonaceous  deposits.  The  most  volatile  con- 
stituents sometimes  escape  at  the  surface  as 
natural  gas.  This  gas  may  accumulate  in  a  porous 
layer  below  some  impermeable  rock,  and  when 
this  cover  is  pierced  by  a  bore-hole  the  gas  escapes 
to  the  surface,  and  affords  a  supply  of  cheap 
heat  and  light.  Oil  shale,  such  as  that  of  southern 
Scotland,  contains  oils  which  have  to  be  driven 
off  by  heating  the  shale  in  large  retorts. 


CHAPTER  IX 

THE    METAMORPHIC    ROCKS 

IN  addition  to  the  ordinary  primary  and  secondary 
rocks  there  is  a  group  intermediate  between  them 
and  combining  some  of  their  characteristics. 
Thus  this  group  includes  rocks  which,  like  those 
of  the  primary  division,  are  composed  of  crystal- 
line material,  and  are  unf ossilif erous ;  they  agree 
with  the  secondary  rocks  in  that  their  constituents 
are  arranged  in  layers.  This  banded  structure 
is,  .however,  not  due  to  original  deposition  of  the 


THE  METAMORPHIC  ROCKS  59 

materials  in  successive  layers.  It  is  sometimes 
due  to  molten  rocks  having  solidified  slowly, 
while  they  have  been  flowing  under  great  pressure. 
It  is  sometimes  due  to  a  rock  having  been  so  altered 
by  heat  and  pressure  that  its  particles  have  been 
completely  rearranged,  all  its  original  constituents 
having  been  converted  into  new  minerals.  The 
new  minerals  have  grown  with  their  longer  surfaces 
parallel,  and  thus  the  rock  breaks  into  slabs, 
like  slate.  As  this  splitting  is  due  to  the  crystal- 
line structure  of  the  rock,  it  is  called  crystalline 
cleavage  or  foliation,  to  distinguish  it  from  the 
splitting  of  slate,  which  is  due  to  slaty  cleavage. 
The  foliated  rocks,  and  those  in  which  the  con- 
stituent minerals  have  been  formed  in  the  rock 
by  the  alteration  of  a  previous  mineral,  constitute 
the  metamorphic  group. 

The  process  of  metamorphism  is  due  to  three 
chief  causes — contact  with  molten  rocks,  deep  sub- 
sidence in  the  earth's  crust,  and  dynamic  action. 
They  are  known  respectively  as  contact-meta- 
morphism,  thermo-metamorphism,  and  dynamo- 
metamorphism.  Contact  -  metamorphism  occurs 
along  the  junction  of  a  molten  rock  mass  with 
some  older  rock,  which  is  altered  by  the  heat. 
Thus  a  granite  mass  that  has  been  forced  into  a 
series  of  older  rocks  is  usually  surrounded  by  a 
zone  of  altered  rocks,  known  as  its  contact  aureole. 
Thermo-metamorphism  is  probably  due  to  the 
rocks  of  a  wide  region  sinking  so  deeply  below  the 
earth's  surface  that  they  are  intensely  altered  by 
the  combined  effect  of  the  intense  heat  and  pressure. 
As  this  action  affects  all  the  rocks  in  a  wide  area, 
it  gives  rise  to  regional  metamorphism. 

Dynamo-metamorphism  happens  during  the 
great  earth  movements  that  accompany  the  for- 


60  GEOLOGY 

mation  of  mountain  chains.  Bands  of  rock  are 
ground  to  powder  along  the  planes  of  movement, 
and  others,  perhaps  less  severely  crushed,  have 
their  constituents  recrystallised  by  the  heat. 

The  term  "  metamorphism  "  is  strictly  confined  to 
processes  which  only  rearrange  the  constituents 
of  a  rock  and  do  not  add  to  or  remove  any  of 
them.  Metasomatism,  on  the  other  hand,  in- 
volves an  actual  change  in  the  constituents  of 
the  rock.  There  is  usually  a  replacement  of  one 
or  more  of  the  original  constituents  by  fresh 
material;  the  process  may  be  due  either  to  hot 
solutions  soaking  through  the  rock,  or  to  the 
absorption  by  one  rock  of  some  constituent  from 
another.  A  sandstone,  consisting  only  of  silica 
and  alumina,  cannot  produce  a  mica-schist ;  but  if 
alkalies  be  added  to  it  in  solution  from  an  adjacent 
granite,  then  the  sandstone  may  be  altered  into 
mica-schist.  Metasomatism  plays  a  very  important 
part  in  the  formation  of  mineral  lodes. 

The  chief  metamorphic  rocks  are — 

(1)  Gneiss,  which  consists  of  the  same  minerals 
as  granite,  arranged,  however,  in  parallel  layers 
instead  of  irregularly.     Syenite-gneiss  and  gabbro- 
gneiss  are  syenites  and  gabbros  with  the  same 
parallel  arrangement. 

(2)  Schists    have     their    minerals    in    thinner 
layers  than  gneiss,  so  that  the  rock  has  a  slate- 
like    aspect.     The   schists    are    known    as    mica- 
schist,  hornblende-schist,  etc.,  according  to  their 
characteristic  mineral. 

(3)  Marble   is   a   metamorphic   rock    which    is 
usually  non-foliated;    it  is  an  altered  limestone. 
The  term  "  marble  "  is,  however,  often  applied  to 
any  rock  that  can  be  easily  cut  and  polished. 

(4)  Quartzite  is  an  altered  sandstone. 


PART  III 

PHYSICAL  GEOLOGY 

CHAPTER  X 

THE    WEARING    AWAY    OF   THE    LAND 

THE  first  natural  impression  formed  of  a  moun- 
tain or  a  hill,  when  we  stand  beside  it,  is  one  of 
vast  size  and  apparent  permanence.  The  smooth 
slopes  with  their  thick  carpet  of  old  turf  and  the 
ribs  or  crags  of  rocks  that  may  here  or  there 
break  the  evenness  of  the  surface,  have  the  aspect 
of  antiquity  and  durability.  Though  it  is  clear, 
even  on  a  casual  inspection,  that  the  hills  have 
changed,  gradually  growing  into  their  present 
shapes ;  though  we  can  see  the  gullies  on  the  hill- 
side are  still  being  deepened  and  widened;  and 
the  very  existence  of  the  rough  crags  implies  the 
wearing  away  of  the  softer  rocks  beside  them, 
yet  these  processes  are  so  slow  that  they  appear 
negligible.  Even  the  earthworks  of  prehistoric 
man  on  the  hilltops,  and  the  tracts  and  paths  that 
led  to  his  ancient  camps,  are  often  still  distinct 
and  have  not  been  worn  away,  though  the  rock 
may  be  as  soft  as  the  chalk  of  the  English  Downs ; 
while  the  quicker  changes  wrought  by  man  in 
railway  cuttings  or  prolonged  quarrying  are  so 
insignificant  compared  with  the  bulk  of  the  hills, 
that  there  seems  little  exaggeration  in  the  poetic 
language  which  speaks  of  the  "  everlasting  hills." 
This  expression,  however,  judges  the  hills  only 
by  comparison  with  the  short  life  of  man,  and 
not  with  the  slow  processes  of  geological  change, 
61 


62  GEOLOGY 

by  which  a  country  has  grown  into  its  present 
form.  Study  of  the  structure  of  the  mountains 
soon  dispels  any  idea  of  their  immutability  and 
immortality,  and  shows  that  they  are  only  the 
remnants  of  once  larger  rock  masses;  and  moun- 
tains usually  give  more  striking  evidence  of 
change  than  of  indestructibility.  The  geological 
structure  of  many  mountains,  as,  for  example, 
Snowdon,  shows  that  they  are  the  last  fragments 
of  a  great  fold  of  rock;  the  mountain  summit 
was  originally  the  bottom  of  a  valley,  between 
hills  which  have  been  all  removed,  and  replaced  by 
valleys  so  cut,  that  what  was  originally  the  floor 
of  the  valley  has  been  left  as  a  mountain  summit. 

The  North  Downs,  south  of  London,  show  a  good 
example  of  this  process  (Fig.  6) .  Looking  southward 
from  their  summit  across  the  valley  of  the  Weald, 
there  may  be  seen  another  line  of  chalk  hills, 
forming  the  South  Downs.  The  floor  of  the  valley 
between  is  occupied  by  rocks  older  than  the 
chalk;  these  rocks  have  been  exposed  by  the 
removal  of  the  thick  sheet  of  chalk  that  once 
rose  in  a  vast  arch  across  the  country  that  is  now 
occupied  by  the  broad  valley  of  the  Weald. 

Geology  shows  that  most  hills  are  only  frag- 
ments of  once  larger  hills.  The  agents  that  have 
worn  them  down  to  their  present  size  work 
slowly;  but  they  effect  great  changes  owing  to 
their  untiring  and  often  unceasing  attack.  The 
whole  land  is  crumbling,  for  while  some  agents 
cause  the  decay  of  any  rocks  exposed  on  the 
surface  of  the  earth,  other  agents  remove  the 
decayed  material,  uncovering  deeper  layers  which 
are  destroyed  in  turn.  This  process  is  known 
as  Denudation. 

The  chief  agents  of  denudation  are  air  and  water, 


THE  WEARING  AWAY  OF  THE  LAND      63 


and  the  changes  in  temperature  between 
night  and  day.  Rocks  exposed  to  the 
atmosphere  suffer  from  the  changes  caused 
by  expansion  and  contraction,  as  they  are 
warmed  in  the  daytime  and  cooled  at 
night.  Granite  readily  cracks  when  it  is 
alternately  expanded  and  contracted  by 
heat,  for  the  three  minerals  of  which  it  is 
composed  expand  at  different  rates,  and 
thus  the  rock  is  torn  by  numerous  fissures. 
In  dry,  hot  climates  a  continuous  crack 
forms  parallel  to  the  surface,  causing  the 
outer  part  of  the  rock  to  break  off  in  large 
thin  slabs ;  this  flaking  causes  granite  in 
tropical  and  sub-tropical  countries  to  wear 
into  even,  dome-shaped  masses,  which 
have  often  been  mistaken  for  the  smooth, 
rounded  hummocks  formed  by  ice.  The 
shattering  of  rocks  by  sudden  cooling  is 
practised  in  primitive  quarrying  and 
mining  by  people  who  have  no  explosives 
or  machine  tools.  A  fire  is  lighted  on 
the  surface  of  rock  in  a  quarry,  and  the 
heat  cracks  off  a  large  slab.  Or  a  fire  is 
placed  at  the  end  of  a  tunnel  in  a  mine, 
and  the  heated  rock  is  suddenly  cooled 
by  being  drenched  with  water,  which 
causes  the  rock  to  crack  and  fly  to  pieces. 
In  countries  with  a  moist  atmosphere  the 
rocks  are  cooled  and  warmed  more  slowly, 
so  that  they  are  less  severely  cracked ;  but 
the  rain  water  soaks  into  pores  and  cavi- 
ties in  the  rocks,  and  when  the  rock  is 
cooled  below  freezing-point  (32°  F.)  the 
water  is  frozen,  and  its  expansion  forces 
off  flakes  of  the  rock.  Such  flakes  are 


f 

2  ° 


•^        |_, 

CJ     H 


OT3 


1 


H 

5 


64  GEOLOGY 

called  frost  flakes.  Similar  flakes,  due  to  sudden 
heating  by  the  sun,  are  known  as  insolation  flakes. 
Either  process  slowly  breaks  off  the  outer  layer 
of  a  rock  and  exposes  a  fresh  surface  to  attack. 

A  still  more  powerful  effect  is  produced  by  the 
freezing  of  water  in  the  large  cracks  and  fissures 
in  rocks.  The  expansion  of  this  water  may  force 
a  large  block  of  rock  out  of  a  cliff  face,  and  its 
fall  may  smash  and  scatter  the  loose  material  at  the 
foot  of  the  cliff.  Alpine  climbers  have  to  avoid 
places  swept  by  such  rock -falls.  They  occur 
mostly  either  shortly  after  sunset,  when  the 
water  freezes  in  the  cracks,  or  after  the  sun  has 
begun  to  warm  the  cliff  face  in  the  morning, 
when  the  melting  of  the  ice  releases  blocks  of  rock 
that  have  been  pushed  forward  by  the  freezing 
water  of  the  previous  evening. 

Rocks  are  also  destroyed  by  dust  hurled  against 
them  by  the  wind.  This  action  is  similar  to  that 
of  the  sand-blast;  this  machine,  by  a  current  of 
air,  flings  a  jet  of  sand  against  a  surface,  which  is 
thus  rapidly  worn  away.  A  jet  of  soft  powder 
will  wear  away  a  much  harder  material.  Wheaten 
flour  blown  against  glass  will  cut  it  away,  just  as 
the  rubbing  of  a  rope  will  cut  a  groove  in  sand- 
stone, as  may  often  be  seen  in  walls  beside  a 
canal  where  the  tow-ropes  rub  perpetually. 

The  wind  will  sometimes  blow  with  high 
velocity;  even  in  a  gentle  breeze  the  air  ad- 
vances eighteen  miles  an  hour,  and  in  a  gale  up 
to  sixty-five  miles,  and  in  a  hurricane  ninety 
miles ;  and  where  the  wind  is  confined  in  a  narrow 
valley,  its  speed  may  be  much  increased.  The 
air  swiftly  bears  along  hard  grains  of  sand  and 
hurls  them  against  bare  rock  surfaces,  which  are 
slowly  worn  away.  As  most  of  the  dust  is  carried 


THE  WEARING  AWAY  OF  THE  LAND    65 

along  close  to  the  ground,  sand  erosion  is  most 
effective  at  the  foot  of  a  cliff,  which  is  thus  under- 
cut, so  that  its  upper  part  falls  over.  Sand 
erosion  has  its  greatest  effect  upon  materials 
that  are  rigid  and  inelastic ;  for  soft  elastic  material 
will  yield,  and  on  its  rebound  fling  off  the  sand 
grain,  whereas  a  harder,  less  elastic  material 
will  be  cut  away.  The  removal  of  the  less  elastic 
material  in  a  cliff  or  in  a  rock  leaves  an  irregular 
surface,  which  is  open  to  attack  from  other  agents. 

Water  is  the  most  powerful  agent  in  denuda- 
tion; it  works  in  the  form  of  rain,  of  rivers  and 
seas,  and  also,  when  frozen,  of  ice.  Rain,  falling 
on  rocks,  soaks  into  pores  and  crevices,  and  if 
this  water  freeze  at  night,  its  expansion  tends 
to  shatter  and  disintegrate  the  rock.  In  addition 
to  thus  forcing  the  particles  of  the  rock  apart, 
the  water  has  also  a  solvent  effect.  Rain  water 
always  contains  some  gases  which  it  has  obtained 
from  the  atmosphere,  and  they  combine  with  the 
constituents  of  the  rocks,  thus  forming  new 
materials.  The  chief  gases  are  oxygen  and  car- 
bonic acid,  which  combine  with  the  constituents 
of  the  rocks  to  form  oxides  and  carbonates.  This 
chemical  change,  by  causing  expansion,  therefore 
helps  the  crumbling  of  the  rock.  Some  of  the 
constituents  of  a  rock  may,  on  the  other  hand, 
be  dissolved  and  removed  in  solution,  leaving 
the  rock  more  porous,  and  still  more  open  to  the 
entrance  of  air  and  water. 

The  water  entering  the  rocks  by  numerous 
openings  on  the  surface  unites  as  it  percolates 
downward,  until  it  may  finally  form  subterranean 
streams;  and  if  they  reach  beds  of  any  soluble 
rock,  such  as  limestone  or  beds  containing  salt 
or  alum,  these  materials  are  carried  away  in  solu- 
tion, leaving  caves  and  empty  spaces. 

E 


66  GEOLOGY 

The  rain  that  does  not  percolate  underground 
flows  over  the  surface  and  collects  into  rills  of 
water,  and  they  in  turn  unite  into  streams.  The 
streams  unite  to  form  rivers.  The  rivers  are 
probably  increased  by  receiving  water  that  has 
percolated  underground,  and  discharges  through 
springs  on  the  bed  of  the  river. 

Running  water,  alike  in  rills,  streams,  and  rivers, 
attacks  and  wears  away  the  rocks  over  which  it 
flows.  It  attacks  them  in  two  ways — mechani- 
cally and  chemically.  The  chemical  process  is 
the  solution  of  the  soluble  constituents  of  the 
rock,  as  sugar  can  be  separated  from  sand  by 
washing  a  mixture  of  the  two  with  water.  The 
mechanical  action  is  the  removal  of  the  material 
bodily,  as  a  stream  carries  away  leaves  and  twigs. 
The  mechanical  action  is  aided  by  the  chemical, 
which,  by  removing  the  cement  from  a  rock, 
causes  its  grains  to  fall  apart,  and  they  can  then 
be  removed  mechanically.  Rain  washes  away 
grains  of  sand  and  clay,  so  that  the  water  becomes 
muddy. 

The  excavating  action  of  streams  and  rivers  is 
conveniently  divided  into  two  kinds — the  wearing 
away  of  the  bed  of  the  river,  which  is  known 
as  corrosion,  and  the  wearing  away  of  the  banks, 
which  is  known  as  erosion.  Corrosion  cuts  a 
narrow  gorge  (Fig.  n),  which  erosion  widens  into  a 
broad  valley.  The  rate  of  both  processes  depends 
largely  on  the  swiftness  of  the  current,  for  upon 
that  depends  the  amount  of  material  carried  by 
the  river.  Pure  water  has  very  little  power  of 
wearing  away  hard  rocks,  but  a  river  loaded  with 
sand  soon  wears  away  the  rocks  over  which  it 
flows.  Clear  river  water  flowing  quickly  across 
clay  or  soft  rocks  will  rapidly  corrode  them, 


THE  WEARING  AWAY  OF  THE  LAND    67 


for  the  water  softens  the  surface  of  the  clay  or 
the  cementing  material  in  the  soft  rock,  and 
ascending  currents,  caused  by  the  eddies,  will 
uplift  and  remove  the  loosened  material. 

A  rapid  current  can  carry  more  and  coarser 
material  than  a  slow  current.  Hence,  the  quicker 
the  current,  the  greater  its  corroding  power. 
For  when  coarse  material  is  carried  across  rocks 
in  the  river  bed,  the  sand  grains  and  pebbles  act 
like  the  teeth  on  a  file.  This  rasping  action  is 
very  effective,  as  the 
river  is  always  at  work, 
and,  owing  to  its  plas- 
ticity, its  teeth  are 
brought  in  contact  with 
all  parts  of  its  bed, 
however  irregular  it 

may  be.  Sections  through  River  Valley. 

Corrosion  is  also  aided    The  uPPer  Yalley  [s  a  g°rg.e 

i        n       ,•  i  •   r      due  to  corrosion ;    the  lower  is 

by  floating  trees,  which    a  valley  widened  by  erosion, 
strike  with  great  force 

against  rocks  on  the  river  bed.  Trees,  of  which 
the  wood  is  heavier  than  water,  as  is  the  case 
with  many  of  the  trees  of  Australia,  or  which 
are  weighted  by  masses  of  earth  and  stones 
attached  to  their  roots,  sink  in  deep  rivers  and, 
drifting  slowly  along,  tear  up  the  river  bed. 

Rivers  often  also  deepen  their  valleys  by  the 
action  of  waterfalls.  A  waterfall  occurs  where 
a  bar  of  hard  rock  crosses  a  stream.  This  rock 
is  cut  away  more  slowly  than  the  soft  rocks 
below  it,  and  thus  it  projects  as  a  ledge  over 
which  the  water  rushes  as  a  cataract  or  leaps  as 
a  waterfall.  The  splash  of  the  water  at  the  foot 
of  a  waterfall  wears  away  the  beds  beside  it,  and 
the  upper  part  is  left  undercut.  Blocks  of  the 


68  GEOLOGY 

projecting  rock  fall  away,  and  may  be  flung  by 
the  water  against  the  foot  of  the  cliff.  Every 
waterfall  is  being  thus  cut  backward,  leaving  a 
narrow  gorge  or  canyon  below  the  fall.  The 
Niagara  Falls  are  moving  upstream  on  an  average 
of  over  four  feet  a  year.  Every  river  will  in 
time  cut  away  the  hard  bars  of  rock  that  form 
waterfalls,  which  are  thus  destroyed.  There  are 
no  rapids  or  waterfalls  on  an  old  river,  as  they 
have  been  cut  away.  The  mere  existence  of 
waterfalls  on  a  river,  therefore,  shows  that  there 
has  been  some  great  change  in  the  geography  of 
the  country  at  a  date  which  is  geologically  recent. 

Deep  hollows  are  often  cut  out  below  waterfalls, 
or  in  parts  of  a  river  where  the  current  is  especially 
rapid,  by  the  formation  of  pot-holes.  A  pot- 
hole is  formed  where  a  large  stone  is  caught  in  a 
hollow  on  a  river  bed.  If  the  current  causes  the 
stone  to  spin  round,  it  will  wear  away  the  under- 
lying rock.  The  stone  acts  like  a  drill  and  bores 
its  way  downward  until  it  is  all  worn  away. 
Fresh  stones  will  be  washed  into  the  pot-hole, 
and  the  swirl  of  the  water  in  the  deepening  cavity 
continues  the  process  (Fig.  n).  Pot-holes  usually 
occur  in  groups,  and  the  rock  of  the  river  bed  may 
in  time  be  honeycombed.  As  the  pot-holes  are 
widened  their  walls  break  down,  and  thus  the 
river  bed  is  lowered.  According  to  some  authori- 
ties, the  formation  of  deep,  river-cut  valleys  in 
hard  rocks  is  mainly  due  to  pot-hole  action. 

Corrosion  deepens  a  river  bed  until  its  incline 
is  so  gradual  that  the  water  flows  along  too 
sluggishly  to  wear  its  bed  away  any  deeper,  and 
the  current  cannot  even  remove  the  material  that 
falls  on  to  the  river  bed  from  the  banks,  or  is 
deposited  on  it  after  a  flood.  A  river  in  this 


THE  WEARING  AWAY  OF  THE  LAND  69 

condition  has  reached  its  "  base  level  of  cor- 
rosion." Rivers  usually  flow  most  quickly  near 
their  source,  among  the  hills,  where  the  slope  of 
the  country  is  steepest;  and  they  flow  more 
slowly  across  the  plains  in  the  lower  part  of  their 
course.  Hence  the  base  level  of  a  river  is  usually 
a  long  curve,  steep  at  first,  and  then  becoming 
gradually  horizontal,  like  the  curve  of  a  piece  of 
string  when  one  end  is  lifted  up  and  the  other  is 
lying  upon  a  table.  That  is  the  curve  of  the 
base  level  in  a  simple  river ;  but  if  the  river  enters 


Fig.  8. 

Longitudinal  Profile  along  a  River  Valley.  The  river  has 
reached  its  base  level  ft1,  due  to  the  rock  barrier  that  upholds  the 
lake;  after  escaping  from  the  lake  it  reaches  a  second  base 
level,  bz. 

a  lake  or  a  plain,  and  then  flows  out  again,  it  will 
have  one  base  level  curve  above  the  lake  and 
another  below  it  (Fig.  8). 

As  soon  as  the  base  level  has  been  reached,  a 
river  will  begin  to  attack  its  banks.  The  force  of 
the  current  is  directed  first  against  one  place 
and  then  against  another,  now  against  one  bank 
and  then  against  the  other.  The  river  washes  away 
the  foot  of  a  bank,  then  the  upper  part  falls  into 
the  river.  This  obstruction  diverts  the  current 
and  directs  it  against  another  part  of  the  bank. 
Meanwhile  wind  and  rain  are  attacking  the  river 
banks,  wearing  away  the  upper  part  of  the  slope, 
and  transforming  a  narrow,  cliff-bounded  gorge 
to  a  broad  valley  with  sloping  sides.  At  the  same 
time,  tributary  streams  have  been  cutting  valleys 


70  GEOLOGY 

through  the  adjacent  country  and  lowering  their 
own  beds  to  their  base  level,  which  is  determined 
by  that  of  the  main  stream.  Little  by  little  the 
country  alongside  the  river  is  lowered  to  a  gentle 
slope.  The  waterfalls  all  disappear,  because  the 
bars  of  rock  which  formed  them  are  cut 
through;  the  lakes  are  filled  up  by  silt,  or  else 
drained  by  the  barrier  that  upheld  the  water 
having  been  cut  away;  hills  with  steep  cliffs  are 
worn  into  hills  with  smooth  slopes,  like  rounded 
downs;  and  finally  in  the  course  of  ages  the  hills 
are  all  worn  away,  and  the  whole  country  is  re- 
duced to  a  plain  with  only 
sufficient  slope  to  allow 

the  water  to  drain  away. 

Fig.  9.  Such  a  slope  is  so  gradual 

Section  along  an  old  Pene-    that  it  is  barely  percep- 

plane  which  has  been  uplifted  tiv>ip  .  +VIP  ™nnfrw  iViPn 
and  destroyed  by  the  forma-  tlble  '  tn6r  country  tnen 
tion  of  valleys  across  it.  Consists  of  a  plain  rising 

slowly  from  the  sea  level 

or  from  the  rivers  to  the  watershed,  which  is  the 
line  separating  the  waters  flowing  into  adjacent 
river  systems.  As  such  a  river-cut  plain  is  nearly 
a  plane,  it  is  called  a  peneplane  (from  the  Latin 
words  pene,  almost,  and  planus,  a  plane) ,  or  pene- 
plain, just  as  land  that  is  nearly  an  island  is  called 
a  peninsula. 

The  tendency  of  river  action  is  to  reduce  all 
the  land  to  such  peneplanes,  and  rivers  are  so 
powerful  and  constant  in  their  action  that  the 
planing  down  of  the  land  takes  place  with  sur- 
prising rapidity.  The  Mississippi  is  said  to  be 
lowering  the  average  level  of  its  whole  basin  at 
the  rate  of  an  inch  in  about  375  years. 

The  destruction  of  the  hills  and  high  land  is 
further  aided  by  the  action  of  springs.  Rain 


THE  WEARING  AWAY  OF  THE  LAND    71 

soaks  underground  and  then  flows  in  subter- 
ranean channels;  this  water  moves  so  slowly 
that  it  cannot  remove  much  material  in  sus- 
pension ;  but  the  very  slowness  of  its  flow  increases 
its  opportunity  for  dissolving  any  soluble  material 
that  it  meets  on  its  course.  The  most  widespread 
material  that  is  soluble  in  ordinary  rain  water  is 
carbonate  of  lime,  the  essential  constituent  of 
limestone.  Water  percolating  through  a  porous 
rock  may  remove  all  its  carbonate  of  lime;  the 
rock  is  thereby  weakened,  and  the  residue  may 
collapse.  Water  percolating  through  limestone 
dissolves  some  of  the  rock  along  its  channel, 
leaving  a  long  cavity  or  cave.  The  widening  of 
the  cave  at  length  causes  the  fall  of  the  roof;  and 
the  course  of  the  former  cave  is  marked  by  a 
valley  with  steep,  wall-like  sides.  The  rock  on 
the  floor  of  the  valley  having  been  shattered  by 
its  fall  is  especially  open  to  the  entrance  of  water, 
and  undergoes  solution  at  an  increased  rate.  Caves 
thus  give  rise  to  those  deep  gorges  which  are  the 
most  picturesque  features  in  limestone  districts. 

Underground  water  aids  in  the  widening  of 
valleys  by  causing  landslips.  Water  percolates 
downward  through  rocks  until  it  reaches  an  im- 
permeable layer,  along  the  surface  of  which  it 
moves.  If  this  impermeable  bed  reaches  the 
surface  of  the  ground  on  a  hill-side,  the  water 
discharges  along  it  in  a  line  of  springs.  The 
water,  may  wash  away  the  material  beside  the 
springs  so  that  the  overlying  rocks  project  un- 
supported, and  in  time  they  fall  into  the  valley. 

If  the  rocks  are  sloping  downward  into  the 
valley,  as  in  Fig.  10,  the  underground  water, 
flowing  over  a  bed  of  clay,  makes  its  surface 
so  slippery  that  masses  of  the  overlying  rocks 


72  GEOLOGY 

may  slide  as  a  landslip  into  the  valley.  Land- 
slips sometimes  involve  the  fall  of  such  enormous 
rock  masses  that  they  form  dams  across  valleys, 
and  the  water  collects  behind  them,  forming 
lakes.  Landslip  action  is  especially  rapid  where 
permeable  rocks  rest  upon  a  sloping  surface  of 
clay. 

The  land  is  also  being  constantly  worn  away 


Fig.  10. 

A  Bed  of  Limestone  (/)  resting  on  a  Bed  of  Clay  (c)  above  Sand- 
stone (s).  The  rocks  are  dipping  into  the  valley  (v}.  Water 
percolating  through  the  joints  in  the  limestone  make  the  upper 
surface  of  the  clay  slippery;  and  masses  of  the  limestone  slip 
downward  in  landslips,  s1  is  the  remains  of  a  fallen  mass;  s~  is 
wholly,  and  s3  partly  detached. 

by  the  sea,  which  cuts  back  the  cliffs  along  the 
shore.  The  coast  of  a  country  is  often  formed 
of  a  line  of  cliffs  rising  above  a  beach  of  shingle. 
The  shingle  consists  of  pebbles  or  rolled  fragments 
of  the  rocks  fallen  from  the  adjacent  cliffs.  In 
storms,  the  waves  not  only  themselves  batter  the 
cliff  with  terrific  violence,  but  as  they  break  upon 
the  beach  they  hurl  against  the  cliff  face  the 
pebbles  whose  oft-repeated  blows  help  to  wear  it 
away.  The  battering  action  is  aided  by  an  ex- 
plosive effect.  Air  is  forced  into  any  cracks  in 
the  rocks  and  there  compressed  by  the  blow  of  the 


THE  WEARING  AWAY  OF  THE  LAND    73 

wave.  When  the  wave  falls  back  from  the  cliff, 
the  air  in  the  crevices  suddenly  expands  with 
such  violence  that  blocks  of  rock  may  be  jerked 


Fig.  ii. 

The  Finart  Glen.  A  Stream  Gorge  cut  by  corrosion.  The 
circular  eddy  on  the  left  side  of  the  stream  is  due  to  a  pot- 
hole in  process  of  formation.  The  rocks  are  sandstones  at  the 
base  of  the  Carboniferous  System.  (By  J .  W.  Reach.} 

from  their  place  and  added  to  the  beach  material 
below.  While  the  cliffs  are  being  cut  slowly 
backward,  the  sweep  of  the  surf  to  and  fro  under 
the  influence  of  tide  and  wind  planes  the  shore 


74  GEOLOGY 

to  a  level  platform.  This  platform  is  increased 
in  width  until  it  forms  a  "  plain  of  marine  denuda- 
tion." The  level  surface  may  be  broken  here  and 
there,  where  a  hard  block  of  rock  resists  the  action 
of  the  surf  and  stands  up  as  a  rock  "  stack." 
Some  stacks  are  so  large  that  they  form  islets. 
Both  stacks  and  islets  slowly  crumble  as  the  waves 
undermine  their  cliffs. 

Ice  Action. — Water  also  acts  in  many  parts  of 
the  world  in  the  form  of  ice.  In  the  Arctic  and 
Antarctic  regions  the  surface  of  the  sea  is  frozen. 
Sheets  of  ice  or  "  ice  floes  "  are  thus  formed,  and 
they  may  be  ten  feet  in  thickness.  Vast  fields 
of  such  ice  are  blown  by  the  wind  against  the  land, 
and  as  they  drift  ashore  they  churn  up  the  beach 
materials  and  are  forced  by  the  pressure  of  the 
ice  behind  to  the  height  of  fifty  feet  or  more  above 
sea  level.  The  impact  of  the  grounding  floe  may 
knock  large  blocks  of  rock  off  the  cliffs;  and  as 
the  lower  surface  of  the  ice  becomes  charged  with 
stones  and  dirt  from  the  beach,  it  grinds  away 
and  polishes  the  rocks  along  the  shore. 

The  tropical  and  temperate  regions  of  the  world 
have  too  warm  a  climate  for  ice  to  exist  on  the 
lower  country;  but  as  the  temperature  falls  about 
i°  F.  for  every  300  feet  of  ascent  above  sea  level, 
the  air  on  high  mountains  is  always  cold.  The 
moisture  from  the  clouds  falls  on  them  as  snow 
and  not  as  rain.  Where  more  snow  falls  than  is 
removed  by  evaporation  or  melting,  the  mantle 
of  snow  increases  in  thickness  and  the  lower 
layers  are  by  pressure  converted  into  ice.  Ice 
formed  in  this  way  is  composed  of  small  grains, 
and  owing  to  the  movement  between  these  grains 
the  ice  is  plastic.  Such  ice,  if  formed  upon  a 
mountain  side,  flows  slowly  down  the  slope  just 


THE  WEARING  AWAY  OF  THE  LAND    75 

as  a  block  of  pitch  will  in  the  course  of  months 
flow  down  a  sloping  board. 

As  rain  water  collects  in  streams  and  flows 
along  the  valleys,  so  ice  flows  along  the  valleys 
and  forms  rivers  of  ice,  which  are  known  as  glaciers. 


Fig.    12. 

An  Alluvial  Flat  on  the  site  of  a  former  Lake.  The  lake  was 
formed  by  the  deposition  of  the  bank  of  moraines,  seen  just 
above,  the  middle  of  the  view.  The  moraine  has  been  cut 
through  arid  the  stream  and  the  lake  thus  drained.  The  rocks 
in  the  foreground  are  Old  Red  Sandstone.  (By  J.  W.  Reach.) 

The  ice  may  be  so  thick  and  widespread  that  it 
may  bury  the  whole  of  a  country  beneath  a  con- 
tinuous sheet.  Such  an  ice  cap  now  occupies  the 
interior  of  Greenland,  and  another  covers  the 
Antarctic  continent.  In  Europe  the  ice  occurs  as 
glaciers,  which  usually  flow  as  well-defined  rivers 
of  ice. 

Glaciers  corrode   and   deepen   their  valleys   as 
rivers  do.     Pure  glacier  ice  would  probably  have 


76  GEOLOGY 

little  power  of  wearing  away  rocks,  but  the  lower 
layers  of  a  glacier  are  usually  charged  with  stones 
and  dirt.  Some  of  this  stony  material  falls  on 
to  the  glacier  from  the  sides  of  the  valley,  and 
is  washed  to  the  bottom  of  the  ice  down  deep 
cracks  or  crevasses,  which  are  formed  where  the  flow 
of  the  ice  is  irregular  owing  to  a  bend  in  its  course 
or  a  sudden  increase  in  its  slope.  More  of  the 
stony  material  is  picked  up  from  the  ground 
beneath  the  ice.  The  sole  of  the  glacier  is  there- 
fore rough  owing  to  the  presence  of  these  included 
rocks  fragments;  and  as  they  are  carried  forward, 
they  press  against  the  ground,  dig  into  the  soft 
material,-and  file  away  the  hard  rocks.  The  stones 
in  the  ice  are  scratched  by  being  ..rubbed  against 
the  underlying  rocks  and  against  one  another. 
When  the  glacier  reaches  a  level  where  the  tem- 
perature is  so  warm  that  the  ice  melts  away 
as  quickly  as  it  is  renewed  by  a  flow  from  above, 
the  material  in  the  glaciers  is  deposited  in  heaps 
known  as  "terminal  moraines"  (Fig.  12).  They 
usually  include  a  motley  assemblage  of  the  harder 
rocks  which  the  glacier  passes  in  its  course,  and 
many  of  the  stones  have  ground  and  ice-scratched 
surfaces. 

The  former  existence  of  glaciers  can  be  recog- 
nised in  countries  from  which  they  have  long  since 
disappeared  by  the  presence  of  the  ice-scratched 
stones,  and  of  rock  surfaces  that  have  been 
polished  by  the  passage  of  ice  across  them.  As 
these  humped  and  hollowed  rock  surfaces  have 
been  compared  to  the  curls  of  a  lawyer's  wig,  they 
have  been  called  roches  moutonnees,  from  the 
French  word  moutonner,  to  crimp  or  wave. 

In  the  colder  regions  of  the  earth  the  glaciers 
may  flow  downward  till  they  reach  the  sea,  Vast 


HOW  ROCKS  ARE  DEPOSITED        77 

blocks  of  ice  are  there  broken  off  from  the  glacier 
and  float  away  as  icebergs.  They  may  carry 
boulders  and  quantities  of  earth  and  stones,  and, 
as  the  ice  melts,  drop  them  in  far  distant  localities. 
Icebergs  from  the  Antarctic  have  been  seen  in  the 
South  Atlantic  within  only  a  few  miles  of  the 
tropics,  and  large  icebergs  from  the  Greenland 
seas  drift  into  the  North  Atlantic  as  far  south  as 
the  steamer  route  between  British  ports  and 
New  York.  Large  boulders  transported  to  a 
distance  by  ice,  whether  by  iceberg  or  glacier,  are 
known  as  "  erratics,"  and  they  often  give  valuable 
evidence  as  to  the  former  movements  of  the  ice. 

Glaciers,  like  rivers,  wear  a  country  away. 
They  lower  their  beds  by  corrosion,  and  also 
widen  their  valleys  by  erosion  of  the  banks. 
As  ice  is  less  fluid  than  water,  a  glacier  adapts 
itself  less  readily  than  a  river  to  bends  in  its 
course.  The  glacier  ice  therefore  presses  against 
spurs  that  project  into  its  valley,  and  slowly  cuts 
them  back ;  hence  a  glacier  flowing  into  a  sinuous, 
river  -  cut  valley  tends  to  cut  it  straighter. 
Valleys  that  have  been  occupied  by  glaciers 
resemble  regular,  steep -sided  troughs;  while 
valleys  cut  by  rivers  are  sinuous,  have  numerous 
bends,  and,  unless  young,  have  gradually  sloping 
sides. 

CHAPTER  XI 

HOW    SECONDARY    ROCKS   ARE    DEPOSITED 

THE  materials  obtained  from  the  destruction  of 
the  primary  rocks  are  reformed  into  the  various 
rocks  described  in  Chapters  VI.,  VII.,  and  VIII. 
These  new  rocks  vary  in  character  according  to 
their  deposition,  whether  it  is  on  land,  in  rivers, 
in  lakes,  or  in  the  sea. 


78  GEOLOGY 

SUBAERIAL  DEPOSITS 

The  deposits  formed  on  land  are  of  four  chief 
kinds — Soil  and  Subsoil,  Talus,  Dunes,  and  Loess. 

Soils  and  Subsoils. — The  most  widespread  land 
deposit  is  the  ordinary  soil.  Rocks  exposed  to 
the  action  of  the  atmosphere  and  of  rain  water 
are  gradually  decomposed.  The  soluble  consti- 
tuents may  be  removed  in  solution,  and  the  rest 
is  disintegrated  by  the  oxygen  of  the  air  uniting 
with  some  of  the  constituents  to  form  new  com- 
pounds. The  rock  thus  gradually  crumbles  to 
pieces.  The  entrance  of  the  air  and  water  is  aided 
by  the  action  of  the  roots  of  plants  or  trees,  which 
help  to  force  the  rocks  asunder.  The  exposed 
surface  of  a  bed  of  rock  is  in  this  way  gradually 
broken  up  into  a  mixture  of  decayed  and  partially 
decayed  rock  fragments.  This  layer  forms  the 
subsoil;  it  is  covered  by  a  layer  of  material  still 
more  thoroughly  decayed  by  the  continued  action 
of  the  same  agents,  and  by  that  of  various  animals, 
such  as  worms,  which  swallow  the  earth  and  eject 
it  at  the  mouth  of  their  burrows  as  worm  castings. 
In  some  arid  regions,  where  the  soil  is  too  dry 
for  worms,  their  place  is  taken  by  white  ants 
and  other  insects.  Worms  and  other  burrowing 
animals  help  to  loosen  the  soil,  and  they  add  to  it 
vegetable  matter  carried  beneath  the  surface  to 
line  their  burrows  or  for  food;  they  thus  contri- 
bute the  organic  constituents,  to  which  most 
soils  owe  their  fertility.  Soils  formed  by  the 
disintegration  of  the  solid  rocks  immediately 
below  them  are  called  Sedentary  Soils.  Those 
formed  from  the  decay  of  sheets  of  surface 
materials  that  have  been  deposited  over  the 
solid  rocks  are  called  Transported  Soils. 


HOW  ROCKS  ARE  DEPOSITED         79 

Talus  or  Screes. — In  mountainous  countries 
rocks  fall  from  cliffs,  and  form  an  accumulation 
of  angular  broken  fragments  at  their  feet.  These 
accumulations  are  known  as  screes,  or  talus  banks. 
If  the  slope  of  the  scree  is  steep,  then,  under  the 
influence  of  rain  and  wind,  of  burrowing  animals, 
and  slight  movements  due  to  rock  expansion 
by  heating  during  the  day  and  contraction  by 
cooling  at  night,  the  material  creeps  gradually 
downward  into  the  valley. 

Dunes. — The  wind  blowing  across  level  country 
sweeps  the  loose  material  before  it,  and  may 
pile  it  up,  where  it  is  caught  by  some  obstacle, 
or  perhaps  by  damp  ground,  into  a  line  of  hills 
known  as  dunes.  They  are  usually  composed 
of  sand,  because  the  lighter  particles  are  blown 
further  away,  until  they  fall  into  some  protected 
hollow  or  into  water.  The  wind,  striking  the 
exposed  side  of  a  dune,  drives  the  particles  of 
sand  slowly  up  the  slope,  and  as  they  are  rolled 
over  against  one  another,  they  are  often  rounded 
and  polished  like  microscopic  marbles.  The  sand 
grains  travel  upward  until  they  reach  the  crest 
of  the  dune,  whence  they  fall  down  the  lee  side. 
By  this  process,  sand  is  continually  carried  from 
the  one  side  of  the  dune  to  the  other,  so  that 
the  dune  slowly  moves  forward  in  the  direction 
of  the  prevalent  wind.  They  may  gradually  cover 
fertile  land,  burying  trees  or  buildings  by  their 
advance.  The  movement  of  the  dune  may  be 
stopped  by  planting  it  with  some  kinds  of  grass, 
the  roots  of  which  bind  the  sand  together.  If  the 
dune  cannot  be  thus  checked,  it  may  invade  and 
desolate  a  populous  district. 

Loess. — Fine  particles  of  clay  are  carried  far 
afield  by  the  wind,  and  may  fall  upon  pools 


8o  GEOLOGY 

of  water,  or  lakes,  or  on  the  sea,  and  therein  be 
deposited  as  mud.  But  if  the  wind  gradually 
loses  its.  force,  as  it  travels  across  a  wide  plain, 
then  the  clay  may  be  deposited  on  the  surface 
of  the  land.  It  is  there  usually  mixed  with 
grains  of  sand,  and  thus  forms  a  sheet  of  loam. 
Many  of  the  particles  in  this  deposit  will  occur 
standing  on  edge,  like  cards  that  have  fallen  through 
the  air  on  to  a  sheet  of  mud,  and  the  number  of 
vertical  grains  will  be  increased  by  roots  forcing 
their  way  downward,  and  by  the  fragments  being 
tilted  as  they  fall  through  the  blades  of  grass 
or  branches  of  other  plants.  The  particles  in 
this  wind-deposited  loam  will  therefore  be  very 
irregularly  arranged,  and  be  interlocked  like  a 
felt.  Hence  it  will  break  as  readily  in  a  vertical 
as  in  a  horizontal  plane,  and  though  the  material 
is  soft,  it  may  stand  in  vertical  walls  or  faces. 
This  material  is  known  as  loess.  It  occurs  in 
thick  sheets  on  former  wind-swept  plains.  Ad- 
vantage is  often  taken  of  its  power  of  standing 
in  vertical  walls  for  the  excavation  of  subter- 
ranean dwellings,  as  in  China,  Hungary,  and 
Spain.  The  origin  'of  loess  as  a  wind  deposit 
was  first  suggested  from  the  nature  of  the  fossil 
bones  found  in  it,  as  they  belonged  to  animals 
that  live  on  treeless  plains. 

It  is,  however,  probable  that  the  material 
called  loess  in  some  districts  is  not  a  subaerial 
deposit. 

AQUEOUS  DEPOSITS 

River  Deposits.  —  Rivers  transport  sediment 
blown  into  them  by  the  wind,  or  that  is  washed 
into  them  from  their  banks,  or  worn  away  from 
their  bed.  The  distance  sediment  is  carried  depends 


HOW  ROCKS  ARE  DEPOSITED         81 

upon  its  coarseness  and  weight,  and  on  the  velocity 
of  the  river.  Grains  of  the  heavy  metals,  such  as 
gold  or  tin,  soon  fall  on  to  the  bed  of  even  a 
rapid  torrent;  but  fine  particles  of  clay  may  be 
carried  for  a  long  distance  and  deposited  when 
the  speed  of  the  current  is  checked  by  the  river 
spreading  out  over  a  wider  channel,  or  crossing  a  more 
level  country,  or  entering  a  lake  or  the  sea.  The 
amount  of  material  that  a  river  can  carry  depends 
upon  its  velocity.  The  quicker  the  current,  the 
more  and  the  heavier  the  material  it  can  carry. 
There  is  one  particular  speed  for  every  part  of 
every  river  at  which  it  can  just  carry  its  burden 
of  sediment  without  depositing  any  or  picking 
up  more  from  its  bed  or  its  banks.  The  river  in 
that  condition  is  said  to  have  reached  its  regime. 
As  a  river  does  not  then  destroy  its  banks  or 
block  its  channel  by  the  formation  of  shoals, 
it  is  the  object  of  engineers  in  charge  of  rivers 
or  canals  so  to  regulate  the  currents  that  they 
are  in  this  condition  of  regime.  But  rivers  that 
are  not  artificially  controlled  are  subject  to  con- 
stantly changing  influences ;  they  are  continually 
at  work,  denuding  here  and  depositing  there,  and 
ever  tending  to  shift  the  position  of  the  channel. 

River  Fans. — When  a  mountain  torrent  escapes 
from  a  gorge  or  glen  into  a  wider  valley,  the  speed 
of  the  current  is  reduced  and  the  coarse  boulders 
and  pebbles,  which  are  rolled  down  its  mountain 
bed,  are  piled  up  in  a  bank  or  fan-shaped  heap 
at  the  mouth  of  the  glen:  thus  the  "river  fan" 
may  grow  out  across  the  wider  valley  as  a  huge 
embankment  that  is  constantly  increased  in 
length  by  the  addition  of  fresh  material. 
,  Deltas. — When  a  river  enters  a  lake  or  the  sea, 
its  current  is  lost  in  the  great  body  of  still  water  ; 

F 


82 


GEOLOGY 


hence  the  coarse  material  carried  by  the  river  is 
deposited  around  its  mouth,  while  the  finer 
material  is  carried  further  and  spread  in  a  sheet 
over  the  bed  of  the  lake  or  sea.  The  material 
dropped  at  the  mouth  of  the  river  at  length  forms 
a  delta,  which  may  be  built  up  as  a  series  of  jetty- 
like  processes  on  either  side  of  the  mouth,  as  in 
the  Mississippi,  or  as  a  triangular  sheet  between 
different  branches  of  the  river,  as  in  the  delta 


Fig.  13. 

The  Flow  of  a  River  Current  in  a  Sinuous  River.    The  stream 
is  eroding  its  banks  at  b,  and  depositing  material  at  a. 

of  the  Nile  or  of  the  Danube.  A  river  fan  may  be 
compared  to  a  delta  which  has  been  deposited  on 
land  instead  of  in  water. 

A  river  that  flows  across  a  level  plain  deposits 
the  material  that  it  has  brought  down  from  some 
higher  part  of  its  course  in  the  deeper  or  broader 
reaches  of  the  river,  where  the  current  is  less 
powerful. 

River  action  is  complex,  as  it  is  not  always 
either  depositing  or  wearing  away  material  at 
the  same  point.  A  heavy  flood,  due  to  a  storm 
of  rain,  greatly  increases  the  rate  of  flow  and, 
therefore,  the  denuding  power  of  the  river;  its 


HOW  ROCKS  ARE  DEPOSITED         83 

bed  and  banks  are  worn  away,  and  the  material 
thus  obtained  is  deposited  when  the  flow  of 
the  river  slackens  on  the  abatement  of  the 
flood.  A  river,  moreover,  may  deposit  material 
on  one  side  of  its  channel  while  wearing  away 
its  other  bank.  A  winding  river  throws  its  current 
first  against  one  bank,  and  then  against  the  other; 
the  current  presses  with  most  force  against  the 
bank  on  the  outer  side  of  a  curve;  the  current  is 
less  rapid,  or  there  may  be  an  eddy  with  a  flow 
upstream  round  the  inner  curve.  Every  one  who 
has  rowed  upstream  on  a  sinuous  river  knows 
that  the  current  is  strongest  at  the  points  b  on 
Fig.  13,  and  weakest  at  the  points  a.  In  the 
dead  water  at  the  points  a,  the  river  may  be 
depositing  the  material  that  it  obtained  by  wearing 
away  the  bank  at  the  point  b  further  upstream. 

A  river  crossing  a  plain  may  deposit  sediment 
over  the  whole  of  its  bed,  while  its  banks  are 
being  raised  by  material  caught  by  the  vegeta- 
tion growing  along  the  water's  edge.  The  whole 
channel  of  the  river  is  thereby  gradually  raised, 
until,  like  some  parts  of  a  canal,  it  is  above  the 
level  of  the  surrounding  country  (Fig.  14).  A  shoal 
forms,  perhaps  around  a  tree  that  has  fallen  into 
the  river  and  been  stranded  on  a  shallow;  material 
collects  in  the  still  water  behind  the  obstacle 
and  forms  an  islet.  More  silting  takes  place  in 
the  dead  water  below  the  islet,  which  may  thus 
increase  in  size.  This  obstruction  diverts  the 
current  against  the  bank,  which  is  worn  away 
and  weakened,  until  the  river  bursts  through  and 
floods  the  surrounding  country.  The  river  may 
thus  take  up  a  new  course,  which  it  will  raise  and 
abandon  as  before.  The  repetition  of  this  process, 
through  the  course  of  ages,  forms  a  widespread 


84  GEOLOGY 

plain  of  alluvium,  through  which  the  river  winds 
its  way.  This  level  sheet  is  known  as  the  "  flood 
plain  "  of  the  river.  The  Nile  furnishes  a  well- 
known  illustration  of  a  river  which  has  raised  its 
bed  above  the  level  of  the  surrounding  country. 
When  its  level  rises  in  the  annual  flood,  the  water 
pours  over  the  banks  and  irrigates  the  adjacent 
lower  country. 

The  material  of  flood  plains  is  usually  clay  and 
]oam,    and   their   soil    forms    the   rich   river-side 


Fig.  14. 

Section   across   a    River  Valley  with   the   River   raised   above 
the  Level  of  its  Flood  Plain. 

meadows.  A  river,  however,  that  has  only  just 
emerged  from  rocky,  hilly  country  may  lay  down  a 
flood  plain  of  shingle,  through  which  the  river  flows 
in  shallow,  constantly  changing,  stony  channels. 

Lacustrine  Deposits. — Lakes  act  as  great  settling 
tanks,  which  collect  all  the  materials  carried  into 
them  by  rivers.  Thus  the  Rhone  enters  Lake 
Geneva  as  a  turbid,  muddy  stream,  which  dis- 
colours the  water  of  Lake  Geneva  for  miles  from 
its  mouth;  but  all  the  mud  settles  in  £he  lake, 
and  the  Rhone  flows  out  of  the  lake  at  Geneva 
as  a  river  of  pure,  transparent  water.  The 
material  spread  over  lake-beds  is  usually  fine 
grained,  since  all  the  coarser  material  is  deposited 
in  the  deltas  at  the  mouths  of  the  rivers.  Great 


HOW  ROCKS  ARE  DEPOSITED         85 

lakes,  like  those  of  North  America,  are  subject 
to  storms,  and  the  heavy  waves  falling  upon  the 
shore  grind  the  rock  fragments  lying  there  into 
shingle.  But  lakes  are  not  subject  to  strong 
tides  like  the  oceans;  their  beach  material  is  not 
swept  backward  and  forward  by  the  unceasing 
ebb  and  flow  of  the  tide,  so  that  it  is  more  angular 
than  that  along  a  sea-shore. 

Marine  Deposits. — Deposits  formed  in  the  sea 
are  of  four  main  types.  Along  the  shore,  the 
battering  action  of  the  waves,  and  the  constant 
backward  and  forward  wash  of  the  tide,  grinds 
the  fallen  materials  into  beds  of  shingle  and  sand. 
The  prevalent  deposits  in  estuaries  and  off  the 
mouths  of  rivers  are  formed  of  clay  brought  down 
by  the  rivers  from  the  land.  If  some  fine  clay  be 
stirred  up  in  a  pailful  of  water,  the  water  will 
remain  discoloured  for  some  hours;  but  the 
addition  of  a  spoonful  of  alum  clears  the  water 
at  once,  by  causing  all  the  clay  particles  to  fall 
to  the  bottom  of  the  pail.  Sea  salt  has  a  similar 
action,  though  the  effect  is  slower.  Hence,  as  soon 
as  mud  passes  from  the  fresh  water  of  a  river  into 
the  salt  water  of  the  sea,  the  material  is  quickly 
deposited  by  this  precipitating  effect  of  sea  salt. 

Further  from  the  shore  the  sea  bed  is  covered 
with  a  mixture  of  sand  and  clay,  and  the  shells 
and  skeletons  of  the  various  marine  organisms. 
The  bulk  of  this  material  is  sediment  derived 
from  the  wearing  away  of  the  land.  This  material 
extends  for  some  distance  from  the  coast  over 
slopes  which  lead  from  the  shallow  sea  near  the 
continents  to  the  deep  ocean  floors.  These  beds 
are,  therefore,  described  as  the  "  deposits  of  the 
continental  slope."  Further  from  the  land  there 
is  naturally  much  less  sediment  in  the  sea  water, 


86  GEOLOGY 

and  in  the  centre  of  the  great  oceans  much  of 
the  material  deposited  on  the  ocean  floor  consists 
of  organic  remains  mixed  with  volcanic  dust. 
These  deposits  are  usually  a  soft,  grey  powder, 
known  as  ooze,  which  is  sometimes  formed  of  the 
shells  of  animals  (Foraminifera  and  Radiolaria),  and 
sometimes  of  those  of  minute  plants,  the  diatoms. 
The  material  of  "  the  great  grey  level  plains  of 
ooze,  where  the  shell-burr' d  cables  creep,"  is 
deposited  with  extreme  slowness.  The  deeper 
parts  of  the  tropical  oceans  are  covered  by  a 
widespread  layer  of  red  clay,  the  residue  left 
after  the  washing  out  of  all  the  soluble  consti- 
tuents of  grey  ooze. 

Glacial  Deposits. — In  many  parts  of  the  tem- 
perate regions  there  are  large  areas  covered  by 
a  confused  series  of  deposits  that  have  been  laid 
down  by  the  action  of  ice.  When  a  glacier  melts 
away,  the  mud  and  stones  scattered  through  it 
are  deposited  as  "  moraines "  around  its  edge, 
or  as  sheets  of  sand  and  gravel  laid  down  by 
the  streams  from  the  melting  ice.  Outside  the 
moraines  that  mark  the  former  margin  of  the 
ice  there  are  often  widespread  sheets  of  "  boulder 
clay,"  composed  of  fine  clay  containing  boulders 
that  are  generally  scratched  and  grooved  by  ice 
action.  The  exact  mode  of  formation  of  this 
boulder  clay  has  given  rise  to  prolonged  con- 
troversy; but  it  must  have  been  formed  in 
positions  whence  the  water  produced  from  the 
melting  of  the  ice  could  not  drain  off  quickly, 
so  that  the  light  clay  was  not  carried  away.  The 
sheets  of  boulder  clay  have  probably  been  often 
formed  in  lakes  due  to  the  damming  up  of  rivers 
by  the  ice,  thus  causing  temporary  ice-bound 
lakes.  The  beaches  formed  %by  such  glacial 


ARRANGEMENT  OF  ROCKS  87 

lakes  are  well  known  in  many  places.  The 
famous  "  Parallel  Roads  of  Glen  Roy "  are  the 
shore  lines  of  a  glacial  lake  that  occupied  part  of 
Glen  Spean  and  its  tributaries  near  Ben  Nevis; 
the  different  "  roads  "  were  formed  at  the  suc- 
cessive levels  of  the  water,  as  the  lake  was  emptied 
by  the  melting  of  the  glacier  that  blocked  the  outlet 
of  the  valley. 

CHAPTER  XII 

THE   ARRANGEMENT   OF   ROCKS   IN   THE   FIELD 

AFTER  the  student  has  become  familiar  with  the 
characters  of  common  rocks  recognisable  in  hand 
specimens,  he  may  proceed  to  the  study  of  their 
arrangement  in  the  field  and  see  how  large  masses 
of  these  rocks  build  up  the  crust  of  the  earth. 
The  first  apparent  difficulty  is  that  in  many 
districts  the  rocks  are  scantily  exposed  to  view. 
In  mountainous  countries  there  are  ample  ex- 
posures of  the  rocks  in  cliffs  and  crags,  and  in 
the  beds  of  the  streams  that  tumble  down  the 
hill-sides.  On  the  coast,  the  rocks  are  generally 
well  shown  in  long  lines  of  cliffs.  In  more 
populous  districts,  however,  and  especially  on 
plains,  exposures  of  rocks  are  more  difficult  to 
find,  and  they  are  usually  small  and  scattered. 
The  surface  of  the  country,  when  crossed  by  road, 
railway,  or  field  path,  may  appear  an  unbroken 
expanse  of  meadow,  field,  or  woodland.  The  beds 
and  banks  of  the  rivers  may  be  formed  of  clay 
or  sand,  and  there  may  be  no  waterfalls  or  cata- 
racts exposing  bars  of  rock  across  the  river 
channels.  But  even  in  such  districts  some  rock 
exposures  can  usually  be  found,  and  others  are 
opened  occasionally;  and  the  local  geologist  has 


88 


GEOLOGY 


then  the  opportunity  for  useful  work  by  studying 
them  and  describing  their  evidence.  In  populous 
districts  brickfields  afford  sections  in  the  clays 
and  loams;  quarries  expose  building  -  stone  and 
limestone;  sections  in  the  road  banks  and  the 
cuttings  for  railways  show  the  harder  rocks, 
and  temporary  excavations  for  drains  or  water- 
pipes  show  what  rocks  occur  below  the  streets  of 
cities.  Mines,  bore-holes,  and  wells  often  give 
sections  deep  below  the  surface.  The  evidence 
afforded  by  the  plough  and  by  material  thrown 
out  from  rabbit  burrows  is  also  often  useful. 


Fig.  15- 
Inclined  Strata  dipping  to  the  W. 

It  is  convenient  to  consider  first  the  arrange- 
ment of  the  stratified  rocks,  because  they  are  the 
most  widespread,  and  as  they  provide  the  time 
scale  by  which  the  age  of  the  igneous  rocks  is 
determined.  The  stratified  rocks  are  laid  down 
in  widespread  layers  that  are  usually  deposited 
horizontally.  Most  stratified  rocks  are  laid  down 
on  the  floor  of  the  sea  or  lakes,  so  that  they  are 
originally  spread  out  in  horizontal  layers  or  beds. 
The  surfaces  between  the  successive  layers  are 
known  as  bedding  planes,  and  they  are  usually 
conspicuous  in  any  section  of  stratified  rocks. 
A  sandstone,  when  examined  in  a  quarry,  is 
usually  found  to  have  layers  of  different  qualities, 
and  may  include  seams  of  clay  or  conglomerate. 


ARRANGEMENT  OF  ROQKS  89 

The  bedding  planes  in  rocks  are  not  usually 
horizontal,  for  they  have  been  tilted  and  have 
a  well-defined  slope.  The  angle  between  the 
slope  of  a  bed  and  the  horizontal  plane  is  the 
"  dip  "  of  the  bed.  The  amount  of  dip  can  be 
measured  by  a  "  clinometer,"  of  which  a  simple 
form  is  a  strip  of  wood  bearing  a  semicircular  card 
marked  with  angles.  A  plumb  line  or  pointer 
is  hung  from  the  middle  of  the  upper  edge  of 
the  card,  and  swings  freely  over  the  graduated 
semicircle.  When  the  long  edge  of  the  instru- 
ment is  placed  along  a  bedding  plane  in  a  quarry 
or  cliff,  the  pointer  will  mark  the  number  of 
degrees  which  the  bed  is  inclined  from  the  hori- 
zontal along  that  rock  face;  and  this  amount  is 
the  "  apparent "  dip,  i.e.  the  dip  in  that  one 
direction. 

The  dip,  however,  can  be  most  conveniently 
measured  on  a  sloping  surface  of  rock.  The  clino- 
meter can  then,  be  rested  upon  the  surface  and 
placed  in  the  position  at  which  the  pointer  rests 
furthest  from  the  zero  point  on  the  card.  The 
angle  which  the  pointer  then  indicates  is  the 
greatest  or  "  true  "  dip.  A  line  drawn  along  the 
surface  of  the  bed,  at  right  angles  to  the  direction 
of  the  clinometer,  when  showing  the  true  dip,  will 
be  horizontal;  and  that  horizontal  line  shows 
the  "  strike  "  of  the  bed. 

The  relation  of  dip  and  strike  may  be  illustrated 
by  a  tilted  card;  the  line  having  the  steepest 
slope  on  the  card  is  the  line  of  true  dip,  and  the 
dip  can  be  measured  along  it  by  the  clinometer. 
If  the  tilted  card  be  dipped  into  a  basin  of  water, 
the  line  along  which  the  card  meets  the  surface 
of  the  water  is  the  line  of  strike. 

The   relation   of   dip   and  strike   may   be   also 


90  4          GEOLOGY 

illustrated  by  reference  to  the  ridged  roof  of  a  house. 
The  slates  sloping  down  on  either  side  represent 
the  beds  dipping  in  opposite  directions,  while  the 
level  crest  of  the  ridge  shows  the  course  of  the 
horizontal  line — the  strike.  If  a  terrace  of  houses 


Fig.  1 6. 

A  Faulted  and  False-bedded  Sandstone  on  the  shore  of  Arran. 
The  dark  area  in  the  left  upper  corner  is  a  pool  of  water.  (By 
J.  W.  Reach.) 

run  north  and  south,  the  strike  is  north  and  south 
and  the  dip  is  to  east  and  west, 

False  and  Current  Bedding. — Many  rocks  have 
minor  bedding  planes  in  addition  to  the  main 
series,  and  these  minor  planes  are  inclined  to  the 
others,  as  they  are  due  to  the  beds  having  been 
laid  down  on  sloping  surfaces.  As  this  bedding 
was  not  laid  horizontally,  it  is  known  as  false 


ARRANGEMENT  OF  ROCKS  91 

bedding.  Materials  laid  down  upon  steep  slopes, 
such  as  the  sides  of  valleys  or  along  steep  shores, 
will  have  their  bedding  parallel  to  the  sloping 
surface  upon  which  they  are  laid  down.  This 
original  slope  of  the  bed,  given  it  during  its  depo- 
sition, must  be  clearly  distinguished  from  the  slope 
given  to  bedding  planes  by  subsequent  tilting. 
True  bedding  is  always  originally  horizontal. 

If  successive  layers  of  sand  and  clay  be  sprinkled 
over  an  ordinary  basin,  the  layers  on  the  flat 
bottom  of  the  basin  will  have  their  bedding 
horizontal,  while  the  layers  on  the  sloping  sides 
will  be  deposited  with  a  false  bedding  (Fig.  17). 

Sands  laid  down  in  a  tidal  estuary  may  have 
a  very  confused  bedding,  as  the  materials  are  laid 
down  sloping  in  different  directions  owing  to  the 
rapid  changing  of  the  currents.  Such  irregular 
stratification  is  known  as  current  bedding.  There 
is,  as  a  rule,  no  difficulty  in  distinguishing  these 
irregular  types  of  bedding  from  true  bedding. 

Unconformity. — All  the  beds  exposed  in  a  quarry 
or  sea  cliff  may  occur  regularly  one  upon  another, 
like  a  pile  of  books  laid  flat  upon  a  table.  If  so, 
the  beds  are  said  to  be  conformable  to  one  another, 
and  they  were  laid  down  as  a  continuous  series. 
Sometimes,  however,  the  beds  in  one  section  belong 
to  two  series;  the  lower  series  may  have  a 
steep  dip,  and  it  may  be  covered  by  a  series  of 
horizontal  beds.  The  relation  between  the  two 
series  is  like  that  between  books  laid  across  the 
top  edges  of  other  books  standing  vertically  on  a 
shelf.  In  such  a  case  the  two  series  of  beds  are 
unconformable  to  one  another.  An  unconformity 
indicates  that  after  the  deposition  of  the  beds  of 
the  first  series,  they  were  tilted  and  worn  away 
until  a  level  or  nearly  level  surface  was  again 


GEOLOGY 


established.  Then  a  new  series  of  deposits  was 
laid  upon  their  worn,  upturned  edges.  An  uncon- 
formity shows  that  a  considerable  lapse  of  time  has 
intervened  between  the  deposition  of  the  two  series. 


Fig.  17. 

Section  through  a  Basin,  on  which  has  been  spread  Layers  of 
Coarse  Sand,  Fine  Sand,  and  Clay.  The  layers  on  the  middle  are 
horizontal,  those  on  the  sides  have  a  false  bedding. 

Joints. — The  bedding  planes  of  a  rock  are  often 
crossed  by  a  double  series  of  cracks,  by  which  a 


Fig.  i 8. 

Section  of  a  Quarry  showing  a  Double  Unconformity, 
at  li1  and  u2. 

bed  of  rock  is  broken  into  separate  blocks.  These 
cracks  are  known  as  "  joints,"  and  their  existence 
is  of  great  value  in  quarrying.  Joints  are  due  to 
the  shrinkage  of  the  rocks  after  their  formation. 
Most  stratified  rocks,  when  first  deposited,  contain 


ARRANGEMENT  OF  ROCKS  93 

some  water.  As  this  water  is  gradually  removed 
the  material  shrinks,  and  the  cracks  caused  by  the 
shrinkage  are  the  joints.  They  are  due  to  the  same 
process  as  the  cracks  in  mud  on  the  floor  of  a 
dried  pool,  and  as  the  columnar  form  of  starch 
when  starch  paste  slowly  dries.  Stratified  rocks 
and  thick  sheets  of  igneous  rocks  are  usually 
broken,  by  jointing,  into  rectangular  blocks,  but 


Fig.  19. 

A  Section  through  Ingleboro,  in  th  Pennines.  The  Carbon- 
iferous beds  (c)  are  resting  unconformably  on  the  Silurian 
beds  (s). 

when  sheets  of  lava  shrink  by  quick  cooling, 
the  jointing  often  forms  long  six-sided  columns. 
These  columns  are  beautifully  shown  in  basalt,  as 
in  the  well-known  cases  of  Fingal's  Cave  in  Staffa, 
and  the  Giant's  Causeway  in  Ireland.  (See  also 
Fig.  26.) 

Weathering  sometimes  produces  a  variety  of 
jointing  that  breaks  rocks  into  rounded  masses, 
from  which  the  surface  peels  off  in  concentric 
crusts  like  the  layers  of  an  onion. 


94 


GEOLOGY 


CHAPTER  XIII 

THE  DISTURBANCES  IN  THE  ROCKS  OF  THE  CRUST 

THE  most  familiar  features  of  the  earth's  surface 
— its  plains,  valleys,  and  lakes — are  due  to  ordinary 
geographical  agencies  that  we  can  see  in  constant 
work  around  us.  They  also  are  the  cause  of  many 
hills  and  mountains,  which  are  left  as  long  ridges 
and  isolated  peaks  by  the  excavation  of  deep,  wide 
valleys  between  them.  The  major  features  of 


AS  M 

Fig.  20. 
Folded  Strata.      A,  Anticline;   S,  Syncline;    M,  Monocline. 

the  earth's  surface  are,  however,  not  due  to  the 
surface  agencies.  Wind,  rain,  and  weather,  rivers, 
and  sea  affect  only  a  very  shallow  zone.  Their 
influence  is  like  that  of  the  sandpaper  or  the 
final  chiselling  with  which  a  sculptor  finishes  off 
a  piece  of  statuary.  The  bolder  features  of  the 
earth's  surface  are  due  to  deep-seated  forces. 
The  ocean  basins  have  been  sunk  and  continental 
masses  raised  by  movements  due  to  changes  in 
the  interior-  of  the  earth.  When  an  apple  dries 
it  shrivels  up,  and  the  skin  is  accordingly  thrown 
into  a  series  of  wrinkles;  and  if  part  of  the  apple 
becomes  rotten,  the  skin  over  it  will  sink  in. 


DISTURBANCES  IN  THE  ROCKS       95 


Similarly    with    the    earth.     Geological    evidence 

indicates  that  the  internal  mass  is  being  slowly 

reduced  in  size; 

but     the     outer  .-"'       / 

crust    does    not 

the 

and 


Fig.  21. 
Isoclinal  Folds. 


shrink     at 

same    rate, 

as  it  sinks  it  is 

compressed    into 

a  smaller  space. 

The  compression 

throws  the  crust 

into   folds,   as   a 

cloth  is  wrinkled  if  it  be  pushed  sideways  across 

a  table. 

Folds. — The  dip  of  rocks  is  caused  by  a  tilting, 

which  may  be  due 
to  the  sinking  of  a 
tract  of  country,  or 
to  the  earth's  crust 
being  compressed 
laterally,  owing  to 
the  shrinkage  in  the 
size  of  the  earth.  If 
a  piece  of  elastic 
be  stretched  and 
attached  by  its 
two  ends  beneath  a 
sheet  of  cloth,  and 
then  the  elastic  be 
allowed  to  contract, 

the  cloth  will  be  thrown  into  a  series  of  folds.     This 

experiment  illustrates  the  condition  of  the  earth's 

crust.     It  is  being  constantly  pressed  into  a  smaller 

space  owing  to  the  shrinkage  of  the  internal  mass 

of  the  earth,  and  like  the  sheet  of  cloth,  it  yields 


Fig.  22. 

A  Normal    Fault.      The    downthrow 
side  is  on  the  right. 


96 


GEOLOGY 


Fig.  23. 
Three  Step  Faults. 


to  the  compression  by  being  bent  into  folds. 
Each  complete  fold  consists  of  a  ridge  and  a 
trough.  The  ridge-like  folds  are  known  as  anti- 
clines (Fig.  20),  because  in  them  the  beds  are 

dipping  away  on 
either  side  from  the 
central  line.  An 
anticline  is  indicated 
on  a  geological  map 
by  the  symbol 
<—  I — >.  The  trough- 
like  down-folds  are 
known  as  synclines 
(Fig.  20),  because 
the  beds  dip  on  either  side  towards  the  central 
line;  the  geological  sign  for  a  syncline  is  — >]< — . 
In  mountainous  countries,  where  the  rocks  have 
undergone  extreme  lateral  compression,  the  rocks 
are  bent  into 

very     crowded £ L. 

folds,  and  both 
sides  of  an  anti- 
cline or  a  syn- 
cline may  dip  in 
the  same  direc- 
tion as  in  Fig. 
21.  Such  folds 
are  known  as 
isoclines.  A  fold 

produced  by  the  sinking  of  part  of  a  country,  so  that 
part  of  a  sheet  of  rock  remains  at  its  original  level, 
is  known  as  a  monocline  (Fig.  20),  as  it  is  a  fold 
with  only  one  side.  If  an  area  has  been  uplifted 
at  one  point,  the  rocks  are  bent  into  a  dome,  and 
the  beds  dip  in  all  directions  from  its  summit. 
The  corresponding  structure  produced  by  depres- 


Fig.  24. 
A  Trough  Fault. 


DISTURBANCES  IN  THE  ROCKS       97 

sion  around  a  point  is  a  basin,  where  the  beds  all 
dip  towards  the  centre. 

Faults. — The  continuity  of  a  bed  may  be  broken 
by  a  part  of  it  having  slipped  downward  along  a 
fracture;  this  break  in  continuity  is  known  as  a 
fault.  In  a  simple  fault  the  beds  on  one  side 
have  slid  downward  along  the  fracture  or  fault 
plane,  which  is 
usually  filled  with  /p- 

crushed  material 
known  as  "  fault 
rock."  The  rocks 
on  each  side  of 
the  fault  are  often 
scratched  and  Fig.  25. 

polished.    The  side  A  Ridge  Fault, 

of     the    fault     on 

which  the  beds  occur  at  the  lower  level  is  said  to 
be  the  downthrow  side  of  the  fault,  while  the  other 
is  called  the  upthrow  side.  Several  parallel  faults 
with  the  downthrow  always  on  the  same  side  of  the 
fault  form  a  series  of  step  faults.  Parallel  faults 
with  a  downthrow  between  them  form  a  trough 
fault,  and  the  valley  between  them  is  a  rift 
valley.  Parallel  faults  which  leave  a  block  of 
rock  upstanding  between  two  areas  which  have 
subsided  are  ridge  faults,  and  the  block  between 
them  is  a  "  horst." 


VOLCANOES,  EARTHQUAKES,  ETC.     99 
CHAPTER  XIV 

VOLCANOES,  EARTHQUAKES,  AND  EARTH  MOVEMENTS 

THE  sinking  of  the  earth's  crust  as  it  follows 
the  shrinking  interior  causes  many  disturbances 
in  the  stratified  rocks,  which  may  be  still 
further  altered  by  masses  of  hot  molten  material 
being  forced  into  them.  The  sinking  of  the 
crust  exerts  intense  pressure  on  the  under- 
lying area,  and  the  hot  rock  beneath  flows  from 
these  areas  into  those  less  strongly  compressed. 
If^the  molten  rocks  consolidate  at  a  considerable 
depth  below  the  earth's  surface  and  in  vast 
masses,  they  form  plutonic  rocks,  which  often 
occur  in  blocks  hundreds  of  square  miles  in  area. 
A  plutonic  rock  bakes  and  alters  the  rocks 
with  which  it  comes  into  contact,  changing  them 
into  rocks  of  the  metamorphic  group.  Plutonic 
masses  are,  therefore,  often  surrounded  by  a  circle 
of  altered  rocks,  known  as  the  contact  aureole. 

The  edge  of  the  intrusive  mass  is  often  irregular, 
as  it  sends  thick,  tongue-like  projections  into  the 
adjacent  rocks.  These  tongues  may  continue  as 
thin  sheets  which  have  forced  their  way  along 
cracks  or  fault  planes,  in  which  they  consolidate 
and  form  those  sheets  of  igneous  rocks  known  as 
dykes.  The  molten  rock  that  fills  these  dykes 
frequently  finds  its  way  into  the  cracks  formed 
around  a  sunken  block  of  the  earth's  crust;  and 
some  of  the  molten  rock  may  rise  through  these 
cracks  to  the  surface  of  the  earth  and  form  a 
volcano. 

Molten  rocks  generally  contain  water,  and 
when  they  approach  the  surface  the  heated  water 
escapes  as  steam.  The  expansion  of  the  steam 


ioo  GEOLOGY 

helps  the  uplift  of  the  rock,  and  near  the  surface 
the  steam  escapes  with  such  explosive  violence 
that  the  rock  is  blown  into  small  pieces.  They, 
after  a  short  course  through  the  air,  fall  in  a  heap 
around  the  vent;  in  time  they  build  up  a  circular 
hill,  with  a  central  cup-like  hollow  known  as  a 
crater.  The  channel  up  which  the  molten  rock 
rises  is  a  volcanic  "  pipe,"  the  mouth  of  which 
is  the  volcanic  "  vent." 

Volcanoes  were  once  regarded  as  burning 
mountains.  It  was  thought  that  they  were 
formed  by  the  combustion  of  beds  of  coal  melt- 
ing the  overlying  rocks.  The  great  dark  cloud 
which  rises  from  the  volcano  was  regarded  as 
smoke  from  the  burning  material,  and  the  lurid 
glow  at  night  was  ascribed  to  flames.  This  ap- 
pearance of  smoke  is  due,  however,  to  the  presence 
in  the  steam  of  fine  particles  of  volcanic  rock — 
volcanic  dust.  The  glow  is  the  reflection  on  the 
clouds  of  the  molten  rock  in  the  crater,  which 
illuminates  the  clouds  as  the  steam  from  a 
locomotive  is  lighted  up  at  night  when  the  fire-box 
of  the  engine  is  opened. 

As  the  cracks  which  enable  the  volcanic  material 
to  reach  the  surface  are  often  the  results  of  earth 
movements,  it  is  usual  to  find  volcanoes  around 
the  edges  of  sunken  areas.  Thus  Vesuvius,  Etna, 
and  the  Lipari  Islands  occur  beside  that  part 
of  the  Mediterranean  known  as  the  Tyrrhenian 
Sea,  which  occupies  the  site  of  a  former  extension 
of  Italy  that  has  foundered  beneath  the  sea. 

A  volcanic  hill  may  be  built  solely  of  fragments 
which  have  been  shot  up  by  explosions  of  steam — 
volcanic  tuffs  and  agglomerates;  or  of  molten 
rock — the  lava — which  has  overflowed  from  the 
vent ;  or  of  mixtures  of  lava  and  tuff. 


VOLCANOES,  EARTHQUAKES,  ETC.     101 

As  the  forces  below  the  volcano  lose  power  by 
the  eruption  of  the  surplus  material,  the  pipe  is 
closed  by  the  solidification  of  the  rock  in  it.  The 
lava  plug  there  formed  prevents  more  material 
reaching  the  crater.  Any  further  supplies  of 
molten  rocks  are,  therefore,  unable  to  escape  up 
the  old  channel,  and  are  forced  into  cracks  beside 
it;  they  often  form  radial  dykes  cutting  through 
the  volcanic  hill.  After  the  volcano  has  been 
much  denuded,  the  dykes  stand  up  as  vertical 
walls  by  the  washing  away  of  the  softer  beds  of 
tuff.  The  last  trace  of  a  former  volcano  may 
be  the  plug  of  hard  lava  that  closed  the  vent,  and 
is  known  as  a  volcanic  neck. 

There  is  no  volcano  in  the  British  Isles  with  a 
still  existing  crater,  but  volcanic  necks  are  common 
in  many  districts,  as  in  southern  Scotland  and 
the  Scottish  Highlands.  Snowdon,  the  highest  of 
the  Welsh  mountains,  is  a  volcano  in  the  last 
stages  of  decay. 

Earthquakes  and  Earth  Movements. — The  earth 
may  be  regarded  as  a  great  projectile  travelling 
through  space  and  spinning  around  its  axis;  and 
it  consists  of  a  hard,  stony  crust  resting  upon  a 
more  mobile  interior,  which  is  probably  slowly 
contracting.  If  we  watch  a  rapidly-revolving  fly- 
wheel, we  may  see  that  it  is  constantly  quivering. 
The  earth's  crust  is  also  in  a  state  of  continual 
quivering  owing  to  its  high  speed  of  rotation  and  its 
irregular  composition.  The  surface  of  the  earth 
at  the  equator  is  moving,  owing  to  its  rotation, 
at  the  rate  of  1000  miles  an  hour ;  and  the  whirling 
crust  is  built  up  of  masses  of  various  materials 
having  different  strengths;  it  has  a  rough,  irre- 
gular surface,  and  the  distribution  of  weight  upon 
it  is  constantly  altering.  Thus  the  weight  of  an 


102  GEOLOGY 

inch  of  rain  is  60,000  tons  per  square  mile.  Hence 
a  heavy  storm  of  rain  over  a  large  tract  of  country 
adds  a  great  additional  load  to  that  part  of  the 
crust ;  and  a  heavy  burden  on  one  side  of  the  earth, 
without  anything  to  balance  it  on  the  other,  would 
give  the  earth  a  slight  tendency  to  wobble  like  a 
badly-balanced  peg-top. 

The  crust  of  the  earth  is  also  disturbed  by  the 
attraction  of  the  moon  and  the  sun.  Their  attrac- 
tion on  the  water  of  the  sea  causes  its  rise  and 
fall,  in  the  movement  known  as  the  tides;  and 
the  same  force  has  a  slight,  but,  as  has  been 
recently  found,  a  perceptible,  effect  upon  the 
crust  of  the  earth,  which  rises  and  falls  twice  a 
day  like  the  tide. 

Under  these  various  influences  the  whole  crust 
of  the  earth  is  quivering  like  a  fly-wheel.  These 
small  tremblings  of  the  earth's  crust  are  known  as 
earth  tremors. 

In  addition  to  these  slight  movements,  which 
are  perceptible  only  to  very  delicately-mounted 
instruments,  the  earth's  crust  is  shaken  by  violent 
movements  known  as  earthquakes,  which  often 
have  disastrous  effects. 

One  chief  cause  of  earthquakes  is  probably  the 
sinking  of  the  earth's  crust,  to  keep  pace  with  the 
shrinking  of  the  interior.  If  the  rim  of  a  fly-wheel 
be  broken,  the  pieces  are  flung  outward;  but  if  all 
the  parts  of  a  fly-wheel  were  being  pulled  toward 
the  centre  by  elastic  cords,  then  when  the  rim 
cracked  the  pieces  would  be  drawn  towards  the 
centre  until  they  were  jammed  in  new  positions. 
When  any  part  of  the  earth's  crust  is  unsupported 
owing  to  the  shrinkage  of  the  material  beneath,  it 
falls  inward  and  sends  a  wave-like  disturbance  or 
earthquake  through  the  adjacent  part  of  the 


VOLCANOES,  EARTHQUAKES,  ETC.  103 

crust.  The  sharp  jerk  caused  by  an  earthquake 
may  overthrow  buildings  and  destroy  towns;  it 
will  fling  rock  masses  down  from  cliffs,  and  thus 
form  dams  across  streams,  alter  the  courses  of 
rivers,  and  form  lakes.  If  the  earthquake  occur 
below  the  sea,  it  may  cause  a  great  wave  which, 
rushing  on  the  shore,  may  devastate  the  coast. 
As  the  earthquake  passes  outward  from  its  place 
of  origin,  its  strength  gradually  dies  away,  until 
it  may  be  felt  only  as  a  slight  shaking,  shown  by 
the  ringing  of  delicately-hung  bells,  or  by  special 
earthquake-recording  instruments. 

In  addition  to  earthquakes  caused  by  the 
foundering  of  blocks  of  the  earth's  crust,  there 
are  others  due  to  raised  masses  of  rock  tending  to 
slip  toward  any  adjacent  lower  ground;  many  are 
caused  by  masses  of  hiaterial  sliding  down  steep 
slopes  on  the  edges  of  the  continents,  and  others 
are  due  to  volcanic  explosions,  which  may  be  so 
powerful  as  to  shake  the  whole  earth.  Thus  the 
explosion  of  Mount  Pelee  in  the  West  Indies  in 
1902  was  felt  in  Melbourne  eight  hours  after- 
wards as  a  wave-like  movement  of  the  surface. 

Slow  movements  of  the  earth's  crust  cause  the 
tilting  of  wide  areas.  Thus  in  recent  geological 
times  the  region  of  the  great  lakes  of  North 
America  has  been  tilted;  the  country  to  the 
south-west  has  remained  stationary,  while  that 
to  the  north-east  of  Lake  Huron,  Lake  Erie,  and 
Lake  Ontario  has  been  elevated  by  an  uplift, 
which  increases  to  the  north-east.  This  move- 
ment is  shown  by  the  lake  terraces,  which  must 
have  been  horizontal  when  they  were  first  formed, 
but  which  have  now  been  tilted  so  that  they 
slope  downward  from  the  north-east  to  the  south- 
west. 


PART  IV 

HISTORICAL  GEOLOGY 

CHAPTER  XV 

THE    STUDY    OF   FOSSILS 

THE  branch  of  geology  which  deals  with  the 
former  life  of  the  earth  is  known  as  Palaeontology, 
from  the  Greek  words  meaning  "  a  discourse  on 
ancient  beings."  Many  rocks  contain  the  remains 
of  animals  or  plants  that  lived  while  the  rocks 
were  being  deposited.  Such  remains  are  known 
as  fossils ;  sometimes  they  are  the  actual  skeletons, 
or  shells,  or  stems;  sometimes  they  are  only 
traces  of  animals  and  plants,  such  as  casts  of  shells, 
footprints,  tracks  made  by  animals  that  crawled 
over  soft  mud,  or  the  imprints  of  leaves.  The 
careful  study  of  all  these  varied  fossil  remains 
has  three  principal  purposes:  (i)  It  gives  the 
geologist  the  best  means  for  comparing  the  ages 
of  rocks  in  distant  parts  of  the  world.  Rocks 
which  contain  the  same  kinds  of  fossils  were 
formed  at  about  the  same  period.  Thus  the 
limestones  at  Wenlock  in  the  English  Midlands,  in 
the  valley  of  the  Yarra  in  Australia,  and  in  the 
state  of  New  York,  all  contain  similar  fossil  shells 
and  corals;  hence  the  geologist  knows  that  these 
limestones  were  all  formed  at  the  same  time. 
(2)  From  the  fossils  of  the  different  rocks  in  a 
104 


THE  STUDY  OF  FOSSILS  105 

district  the  geologist  learns  which  of  the  rocks 
is  the  older  and  which  the  younger;  thereby 
he  determines  the  succession  and  distribution  of 
rocks  and  the  geological  structure  of  the  district. 
(3)  Fossils  reveal  to  us  the  history  of  life  upon  the 
earth. 

The  study  of  fossils  reveals  to  a  geologist  the 
age  at  which  a  rock  was  formed,  as  an  antiquarian 
learns  from  medals  the  dates  of  ancient  ruins. 
It  was  the  discovery,  by  William  Smith  (1769- 
1839),  that  fossils  can  be  thus  used  as  the 
"  Medals  of  Creation  "  that  gained  for  him  the 
title  of  "  Father  of  Geology "  and  founded 
modern  geology. 

Smith  was  a  land  surveyor  working  in  the 
neighbourhood  of  Bath.  The  country  in  that 
part  of  England  consists  of  a  succession  of  lime- 
stone hills,  trending  roughly  N.E.  and  S.W.  and 
separated  by  valleys,  the  floors  of  which  are 
beds  of  clay.  One  possible  explanation  of  this 
arrangement  might  have  been  that  the  clays 
exposed  in  the  valleys  all  belonged  to  one  con- 
tinuous sheet  that  formed  the  foundation  of  the 
whole  country ;  and  that  the  limestones  were  the 
remains  of  one  overlying  sheet  that  had  been 
broken  up  into  successive  bands  by  the  formation 
of  the  valleys.  This  interpretation  is  illustrated 
by  Fig.  28.  William  Smith,  however,  discovered 
that,  although  the  clays  of  the  various  localities 
are  much  alike  in  their  general  appearance,  each 
band,  L,  F,  O,  and  K,  contains  a  quite  distinct 
assemblage  of  fossils.  Each  of  the  limestones  of 
the  hills,  I,  G,  and  C,  has  also  a  different  set  of 
fossils,  differing  from  one  another,  and  from  those 
in  each  of  the  clay  bands.  Smith,  therefore, 
recognised  that  the  country  is  built  of  seven  beds 


106  GEOLOGY 

instead  of  two;  there  are  four  beds  of  clay  of 
different  ages,  separated  by  three  beds  of  lime- 
stone. The  structure  of  the  country  is  as  repre- 
sented in  Fig.  29,  and  not  as  in  Fig.  28. 

The  beds  are  all  tilted  so  that  they  sink  east- 
ward; hence  a  bore  put  down  at  K  would  go 
through  all  the  beds  of  this  series.  The  bed  L 
was  the  oldest  member  of  the  series,  and  the  other 
beds  had  been  laid  down  one  after  another  over 
it,  in  accordance  with  the  first  principle  of  his- 
torical geology,  viz.  that  in  a  succession  of  de- 
posits the  oldest  occurs  at  the  bottom.  Smith 
subsequently  studied  the  country  to  the  north- 
east of  this  area  in  the  English  Midlands.  There 
he  found  that  crossing  the  country  from  west 
to  east  the  series  begins  with  a  sheet  of  clay 
containing  the  same  fossils  that  he  had  found  in 
the  lowest  and  most  western  bed  L  in  the  series 
near  Bath.  This  bed,  the  Lias,  was  not  followed 
to  the  east  by  the  limestone  I  and  the  clay  F,  but 
by  a  series  of  bands  containing  few  fossils,  and 
these,  as  a  rule,  different  from  those  of  I  and  F. 
Further  east,  however,  there  was  another  sheet 
of  clay  containing  the  same  fossils  as  the  bed  O 
to  the  east  of  Bath.  This  clay  was  succeeded  by 
a  bed  of  limestone  containing  the  same  fossils  as 
C,  and  that  by  clay  with  fossils  of  the  bed  K. 
Still  further  northward,  in  the  district  of  the 
Wash  (Fig.  31),  the  beds  of  clay,  O  and  K,  are 
well  developed,  but  the  limestone  C  is  not  found 
here,  so  that  the  two  clays,  distinct  in  themselves, 
form  one  thick  bed.  The  western  part  of  this 
bed  can  be  recognised  as  the  continuation  of  bed 
O,  and  the  eastern  part  as  the  bed  K,  as  when 
pits  are  dug  in  them  they  yield  the  particular 
fossils  of  these  two  clays.  Still  further  north,  in 


THE  STUDY  OF  FOSSILS 


included  sandstones  and  clays);  O,  Oxford  Clay;  C,  Corallian  Lime- 
stone series;  K,  Kimmeridge  Clay;  N.  S.,  Northampton  Sands;  Est., 
Estuarine  beds  with  some  interstratified  marine  limestone. 


io8  GEOLOGY 

Yorkshire  (Fig.  32),  a  section  across  the  country 
from  west  to  east  shows  the  clay  of  the  Lias  to  the 
west,  succeeded  to  the  east  by  beds  that  had  been 
laid  down  in  estuaries,  with  an  occasional  thin  bed 
of  marine  limestone.  The  estuarine  series  is  suc- 
ceeded by  the  clay  O  (the  Oxford  Clay),  above 
which  occurs  an  important  series  of  coral  lime- 
stones, corresponding  to  the  bed  C  in  the  first 
section;  and  east,  again,  is  the  clay  K. 

The  clays  of  the  Lias  (L) ,  the  Oxford  Clay  (O) ,  and 
the  Kimmeridge  Clay  (K),  each  contains  similar 
fossils,  alike  in  Yorkshire,  the  Midlands,  and  the 
south-west  of  England.  The  Lias  was,  therefore, 
being  laid  down  at  the  same  time  all  across 
England.  As  the  beds  between  the  clays  are 
followed  from  Bath  to  Yorkshire,  they  are  found 
to  differ  both  in  the  nature  of  their  rocks  and 
their  fossils,  because  they  were  laid  down  under 
different  geographical  conditions;  but  owing  to 
the  identity  of  the  fossils  in  the  clays,  the  inter- 
vening beds,  whether  marine  or  estuarine,  can 
also  be  safely  correlated. 

Smith  thus  established  the  fundamental  prin- 
ciple of  geology,  that  rocks  at  distant  localities, 
even  though  laid  down  under  different  geographical 
conditions,  can  be  dated  by  the  use  of  fossils. 

The  study  of  fossils  has,  therefore,  proved  in- 
dispensable to  the  progress  of  geology.  Fossils 
are  collected  where  rocks  are  open  for  inspection 
in  railway  cuttings,  brick  fields,  quarries,  mines, 
or  sea  cliffs. 

Fossil  animals  are  of  more  general  importance 
to  the  geologist  than  fossil  plants,  for  they  are 
usually  most  abundant,  and  they  give  the  best 
evidence  as  to  the  date  of  the  fossil,  and  as  to 


THE  STUDY  OF  FOSSILS  109 

the  geographical  conditions  and  climate  under 
which  it  lived. 

The  animals  of  most  value  to  the  geologist 
are  those  that  have  hard  shells  or  skeletons,  as 
those  in  which  the  body  is  composed  only  of  soft 
materials  seldom  leave  traces  as  fossils.  The 
number  of  fossils  is  now  so  enormous  that  their 
description  and  full  identification  is  usually  left 
to  specialists,  who  each  confine  their  studies  to 
one  group.  The  geologist,  however,  should  be 
able  to  recognise  to  which  group  a  fossil  belongs, 
as  he  can  thereby  often  infer  the  geographical 
conditions  under  which  the  deposit  was  laid  down 
and  its  approximate  age.  Fossils  have  to  be 
first  sorted  into  the  chief  subdivisions  of  the 
animal  or  of  the  vegetable  kingdoms. 

The  animal  kingdom  is  divided  firstly  into  the 
two  sub-kingdoms.  In  the  first,  the  Protozoa, 
each  animal  is  composed  only  of  a  single  cell, 
or  possibly  of  several  similar  cells.  The  animals 
belonging  to  the  second  sub-kingdom,  the  Metazoa, 
are  always  multicellular,  that  is,  they  are  composed 
of  many  cells,  and  different  cells  are  modified  to 
do  different  work;  one  set  of  cells  captures  food, 
another  set  digests  it,  and  another  distributes  it 
through  the  body;  other  cells  serve  as  organs 
of  sense,  and  others  for  locomotion. 

The  unicellular  animals  are  generally  so  minute 
that  they  can  only  be  seen,  or  their  structure 
recognised,  by  the  aid  of  the  microscope;  but 
they  often  live  together  in  such  great  numbers 
that  their  dead  shells  form  great  masses  of  rock. 
Thus  the  Foraminifera  (Fig.  33)  usually  build 
shells  of  carbonate  of  lime,  which  are  littered  over 
the  sea  floor,  forming  widespread  deposits  of  ooze, 
or  collected  along  the  shore  as  beds  of  sand.  If 


no 


GEOLOGY 


these  deposits  are  cemented  they  form  forami- 
niferal  limestone,  which  often  occurs  in  thick, 
widespread  sheets.  The  Foraminifera  contribute 
largely  to  the  building  up  of  chalk,  and  Nummu- 
lites,  a  gigantic  member  of  the  group,  form  the 
Nummulitic  limestone,  which  extends  around  the 
Mediterranean  basin  and  occurs  at  intervals 
across  Southern  Asia. 

The  second  group  of  Protozoa  important  to  the 
geologist    is    that    of    the    Radiolaria    (Fig.    34), 


Fig.  33. 

A  Foraminifera 
(magnified). 


Fig-  34- 
A  Helmet- 
shaped  Radio- 
larian  (greatly 
magnified). 


Fig.  35- 
Sponge  Spicules. 

a.  A  Uniaxial  Spicule. 

b.  A  Six-rayed  Spicule. 


which  have  microscopic  shells  composed  of  silica. 
They  therefore  form  siliceous  ooze,  which,  when 
cemented  into  rock,  forms  beds  of  chert. 

The  Metazoa,  or  multicellular  animals,  are 
divided  into  two  chief  sections.  In  the  first 
section,  the  Ccelenterata,  the  body  consists  essen- 
tially of  a  bag  into  which  the  mouth  opens  directly, 
and  there  is  no  separate  digestive  system.  The 
Ccelenterates  include  the  sponges,  many  of  which 
have  a  skeleton  composed  of  thin  rods  or  spicules 
(Fig.  35)  of  silica,  carbonate  of  lime,  or  a  horny 
material,  chitin.  Sponges  with  siliceous  or  cal- 
careous spicules  contribute  to  the  formation  of 
siliceous  rocks  and  limestone. 


I 


THE  STUDY  OF  FOSSILS 


in 


The  second  group  of  Coelenterates  is  known 
as  the  Hydrozoa,  since  the  most  typical  member 
is  the  common  hydra  that  lives  in  pools  and 
ditches.  Hydrozoa  are  small  individually,  but  they 
often  live  in  large  colonies,  composed  of  very  many 
individuals  protected  by  a  continuous  skeleton. 
One  section  of  Hydrozoa,  the  graptolites  (Fig. 
36),  is  allied  to  the  sea-firs,  which  are 
common  upon  our  coasts.  Each  grap- 
tolite  is  composed  of  a  horny  rod  with 
one  or  more  rows  of  cells  along  it;  and 
in  each  cell  lived  a  small  hydra-like 
animal.  Many  graptolites  sometimes 
grew  from  a  central  float.  The  grap- 
tolites are  an  extinct  group,  and  were 
confined  to  the  Lower  Palaeozoic  period. 
They  used  to  live  floating  in  swarms 
on  the  surface  of  the  sea.  Some  of  ^ 

the    Hydrozoa   have    thick    calcareous      Fig.  36. 
skeletons,  and  form  one  group  of  corals.   The  Lower 

The  ordinary  corals,  however,  belong  Pg[^1°1fea 
to  the  third  group  of  Coelenterates  known  Graptoiite. 
as  the  Anthozoa,  which  includes  the  sea 
anemones  and  their  allies.  The  ordinary  corals 
agree  in  general  structure  with  the  sea  anemones, 
but  differ  by  having  a  calcareous  skeleton.  In  some 
corals  many  individuals  or  polypes  grow  together  in 
great  masses  forming  reefs;  these  corals  can  only 
grow  in  warm  water,  so  they  are  limited  to  shallow 
water  in  the  tropical  seas.  Coral  polypes  that 
live  separately  and  form  single  corals  can  live 
in  colder  waters,  and  are  widely  distributed  in 
the  sea,  both  in  latitude  and  in  depth. 

The  second  section  of  the  Metazoa  forms  the 
group  known  as  the  Coelomata,  because  the  body 
cavity  or  ccelome  is  separated  from  the  tube,  by 


ii2  GEOLOGY 

which  food  passes  into  the  body  and  is  there 
digested.  The  Coelomate  animals  are  divided 
into  those  without  and  those  with  a  backbone 
or  vertebral  column.  Of  the  former  there  are 
four  main  divisions. 

1.  The    Echinoderms,    including    the    starfish, 
sea  urchins  or  sea  hedgehogs,  the  sea  lilies,  and 
sea    cucumbers,    all    of    which,    except    the    sea 
cucumbers,  have  a  well-developed  skeleton.     The 
sea  lilies  or  crinoids  are  particularly  important 
as  limestone-forming  animals. 

2.  The    worms    and    their    allies.     Worms    are 
usually  soft   bodied,  but   some   of   them   live   in 
hard  tubes  composed  of  carbonate  of  lime  or   of 
cemented   sand    grains;     they    either    form    cal- 
careous rocks,  or  help  to  form  cherts.     Allied  to 
the  worms  are  two  important  groups  of  fossils, 
the  Brachiopods  or  Lampshells,  and  Bryozoa.    The 
former  have  a  shell  of  two  pieces  or  valves.     Lamp- 
shells  and  their  allies  have  lived  from  the  earliest 
geological   times    to   the   present,    but   they   are 
comparatively  scarce  in  existing  seas.     They  were 
formerly  so  abundant  that  their  shells  built  up 
thick  sheets  of  limestone,  and  they  are  especially 
useful  in  the  correlation  of  the  older  rocks.     Closely 
allied  to  the  Lampshells  are  the  Bryozoa,  in  which, 
though   the   individuals   composing   the   colonies 
are  minute,  they  form  large  encrusting  sheets  or 
moss-like  growths,  and  their  remains  were  once 
mistaken  for  fossil  seaweeds. 

3.  The   Arthropods,    or   animals   with   jointed 
limbs,  include  the  water  fleas,  sand  shrimps,  wood 
lice,   crabs    and   lobsters,    centipedes,    barnacles, 
spiders,  mites,  insects,  etc.     The  shells  of  these 
animals  are,  as  a  rule,  horny,  and  it  is,  therefore, 
only  in  special  deposits  that  they  are  abundant 


THE  STUDY  OF  FOSSILS 


as  fossils.  The  minute  valves  of  the  water  fleas 
are  often  found  in  great  abundance  in  mud  laid 
down  on  the  floor  of  lakes.  The  trilobites  (Fig.  37) 
are  an  extinct  class  of  Arthropods;  they  ranged 
throughout  Palaeozoic  times.  They  had  a  body 
divided  into  three  lobes,  and  some  of  them  could 
roll  themselves  into  a  ball  like  a  wood  louse,  to 
protect  the  soft  gills  on  the  under-side  of  the  body. 
The  Arthropods  that  live  on  land, 
such  as  scorpions,  millipedes,  centi- 
pedes, spiders,  and  insects,  are  com- 
paratively scarce  as  fossils. 

4.  The  Mollusca,  including  the 
shell-bearing  animals  commonly  de- 
scribed as  "shell-fish,"  is  one  of  the 
groups  of  animals  of  most  importance 
to  the  geologist,  on  account  of  the 
abundance  and  wide  distribution  of 
their  fossil  remains.  The  body  of  the 
mollusc  is  usually  protected  by  a 
shell,  which  may  consist  of  two 
pieces  or  valves,  as  in  the  common 
oyster,  mussel,  or  cockle.  Some 
molluscs,  however,  are  protected  by  a  shell  of  a 
single  piece  (hence  its  name  "  univalve  ")  (Fig.  38) ; 
this  shell  may  be  a  short  cone,  as  in  the  limpet ;  or 
a  long  cylindrical  or  gradually  tapering  tube,  as 
in  the  elephant's  tooth  shell  (Dentalium);  or  a 
tube  coiled  into  a  disc,  as  in  the  common  pond 
snail  (Planorbis),  or  into  a  spiral,  as  in  the  turret 
shell  (Turritella).  In  the  univalve  shells  the  tube 
may  be  open  throughout  its  length,  or  it  may 
be  divided  into  many  separate  chambers,  and  if 
so,  the  animal  that  formed  the  shell  lives  in  the 
last  of  them.  The  chambered  tube  may  be 
straight,  as  in  the  Orthoceras,  or  coiled  into  a 

H 


Fig.  37- 
A  Trilobite. 


H4  GEOLOGY 

disc,  as  in  the  nautilus  and  ammonite.  Extreme 
modifications  of  the  shell  are  found  in  the  bone 
of  the  cuttle-fish  or  the  guard  of  the  belemnite, 
in  which  the  chambered  portion  of  the  shell  is 
reduced  to  a  short  cone  in  one  end  of  a  massive 
cylinder  or  guard  (Fig.  39) . 

The   backboned    animals,  or  Vertebrata,  have 
the  body  supported  by  a  backbone  that  is  corn- 


Fig.  38. 

Four  Univalve  Shells,  a,  a  simple  conical  shell ;  b,  a  tubular 
shell,  such  as  the  elephant's  tooth  shell;  c,  a  coiled  shell,  such  as 
the  common  pond  shell,  from  the  side;  d,  the  same  from  above; 
e,  a  spiral  shell. 

posed  of  a  chain  of  bony  joints.  The  backboned 
animals  are  the  fish,  the  amphibia — including 
frogs  and  newts — the  reptiles,  birds,  and  mammals. 
The  backboned  animals  are  the  most  specialised 
members  of  the  animal  kingdom,  and  they  made 
their  first  appearance  later  in  the  earth's  history 
than  any  large  group  of  invertebrate  animals. 
The  fish  are  the  most  primitive  of  the  backboned 
animals,  and  are  naturally  the  oldest.  They  were 
first  abundant  in  the  Devonian  period,  which  has 
been  described  as  "  the  Age  of  Fish."  Many  of 
the  earliest  fish  were  protected  by  a  bony  external 
skeleton;  of  these  fish  with  bony  armour  only  a 
few  now  survive  in  lakes  and  rivers. 

The  reptiles  are  most  important  in  the  Mesozoic 
age,  which  is  therefore  called  "  the  Age  of 


THE  STUDY  OF  FOSSILS 


Reptiles."  They  then  dominated  both  land, arid 
sea,  and  included  gigantic  land  animals  estimated 
at  about  100  feet  in  length ;  others 
that  lived  in  the  sea,  and  others 
that,  by  a  wing-like  development 
of  the  fore-limbs,  as  in  the  bat,  flew 
in  the  air. 

.  The  mammals  made  their  ap- 
pearance at  the  beginning  of  the 
Mesozoic,  but  they  remained  in- 
significant until  the  beginning  of 
the  Cainozoic  Era.  Then  the 
reptiles  lost  their  supremacy, 
and  the  mammals  increased  in 
number,  size,  and  variety,  and 
they  are  the  most  highly  deve- 
loped and  dominant  of  existing 
organisms. 

The  power  to  refer  a  fossil  to 
its  group  is  of  great  value  to  the 
geologist,  as  he  may  thus  infer 
the  conditions  under  which  the 
rock  containing  it  was  formed. 
The  presence  of  large  massive 
corals  shows  that  the  rock  must  part  found  fossil  is 
have  been  formed  in  a  warm  sea,  the  solid  rod  in  the 

i    -i      -i  ,  i  . .      ,       middle  line  of  the 

and  doubtless  at  a  comparatively  lower  part  of  the 
shallow  depth.    The  occurrence  of  figure,  in  which  it 
thin-shelled,  single  corals  would,  j^E  ^ 
on  the  other  hand,  indicate  that 
the   rock  had   been   formed   in   the  sea,   but   in 
colder  water  and  possibly  at  a  great  depth.     The 
presence  of  Foraminifera  and  Radiolaria  is  proof  of 
a  marine  origin,  unless  they  have  been  washed 
into    the    rock    from   some    older    deposit.      Sea 


n6  . GEOLOGY 

lilies,  starfish,  sea  urchins,  large  chambered  shells, 
lampshells,  or  abundant  Bryozoa  are  also  indica- 
tions that  a  bed  containing  them  was  formed  in 
the  sea.  Shells  may  be  formed  on  land,  in  the 
sea,  or  in  fresh  water;  but  a  comparatively 
elementary  acquaintance  with  Mollusca  enables 
a  geologist  to  infer  from  a  few  well-preserved 
shells  the  conditions  under  which  they  lived. 
The  remains  of  insects,  birds,  or  ordinary  mammals 
indicate  a  deposit  laid  down  either  on  or  near 
land. 

An  elementary  acquaintance  with  fossils  also 
enables  a  geologist  to  tell  the  Group  to  which  a 
fossiliferous  rock  belongs.  Thus  the  discovery 
of  a  trilobite  would  prove  that  the  rock  containing 
it  is  Palaeozoic ;  graptolites  are  clear  evidence  of 
the  earlier  part  of  the  Palaeozoic  Era;  ammonites 
and  belemnites  are  almost  confined  to  the  Meso- 
zoic;  mammals  or  birds  are  characteristic  of  the 
Cainozoic,  though  their  remains  occasionally  occur 
in  the  Mesozoic. 

A  student  can  use  fossils  to  determine  the  age 
of  the  rock  from  which  they  came  with  greater 
precision  if  he  will  divide  them  into  their 
different  classes,  and  then  identify  them  by  com- 
parison with  those  in  a  geological  museum. 


SUMMARY  OF  HISTORICAL  GEOLOGY    117 
CHAPTER  XVI 

SUMMARY   OF   HISTORICAL   GEOLOGY 

THE  history  of  the  earth  is  read  by  the  geologist 
from  the  rocks  of  the  earth's  crust,  and  according 
to  their  evidence  geological  time  may  be  divided 
into  four  main  divisions,  known  as  Eras.1  The 
first  Era  is  often  known  as  the  Eozoic,  because  it 
was  that  of  the  dawn  of  life  upon  the  earth.  There 
is  as  yet  no  general  agreement  amongst  geologists 
as  to  the  classification  of  the  rocks  belonging  to 
this  most  ancient  Era;  but  the  tendency  appears 
to  be  to  divide  them  into  an  older  crystalline  series, 
and  a  younger  series  of  comparatively  unaltered 
sediments.  The  crystalline  or  Archean  series  in- 
cludes the  oldest  rocks  that  the  geologist  can  collect 
in  the  field  and  study  in  his  laboratory.  All  its 
rocks  that  remain  to  us  have  been  subject  to  such 
intense  heat  that  their  constituents  are  chiefly 
crystalline,  and  they  contain  no  recognisable  re- 
mains of  any  life  that  may  have  existed  on  the 
earth  at  the  time  of  their  deposition. 

The  upper  Eozoic  sedimentary  rocks  have 
yielded  a  few  obscure  fossil  remains.  Most  of 
the  rocks  are  sandstones,  shales,  and  slates,  which 
have  not  been  profoundly  altered  by  heat;  it 

1The    International    Geological    Congress    has    recom- 
mended the  following  terms  for  the  divisions  of  the  strati- 
fied rocks  and  their  time  equivalents- 
Group  is  equivalent  to  Era 
System  ,,  Period 

Series  ,,  Epoch 

Stage  ,,  Age 

Thus  the  Silurian  System  is  a  member  of  the  Palaeozoic 
Group,  and  was  deposited  in  the  Silurian  Period.  Used 
technically  these  terms  begin  with  a  capital  letter. 


n8  GEOLOGY 

includes  a  vast  series  of  comparatively  unaltered 
sediments,  intermediate  between  the  Palaeozoic 
and  the  Archean  schists  and  gneisses. 

The  second  great  Era  is  known  as  the  Palaeozoic, 
or  the  Era  of  ancient  life,  for  its  rocks  contain 
remains  of  the  oldest  and  most  primitive  animals 
and  plants  of  which  we  have  any  satisfactory 
information. 

The  third  Era  is  the  Mesozoic,  or  the  Era  of 
middle  life.  The  fourth  is  the  Cainozoic,  or  the 
Era  of  modern  life,  during  which  the  earth  was 
inhabited  by  animals  of  modern  types,  some 
of  which  were  the  near  ancestors  of  those  still 
living;  it  is  conveniently  regarded  as  including 
the  present  time,  though  geological  history  ends 
with  the  beginning  of  the  records  written  by  man. 

The  Archean  rocks  may  be  divided  into  two 
chief  divisions.  The  lower  rocks  are  in  the  main 
coarsely  crystalline,  and  they  inchide  a  complex 
series  of  gneisses,  schists,  and  igneous  rocks.  The 
upper  division  consists  of  fine-grained  gneisses  and 
schists,  and  of  quartzites  and  crystalline  lime- 
stones ;  their  arrangement  in  the  field  resembles 
that  of  an  ordinary  stratified  series.  As  the 
coarsely  crystalline  rocks  lie  below  the  less  crystal- 
line, they  were  naturally  at  first  regarded  as  the 
older;  but  they  have  been  proved  in  some  cases 
to  be  intrusive  rocks  forced  into  .the  overlying 
series,  and  are,  therefore,  the  younger  in  age. 
The  rocks  of  the  upper  or  less  crystalline  division 
are  principally,  no  doubt,  a  series  of  sedimentary 
rocks  that  have  been  metamorphosed.  The 
Archean  rocks  in  several  parts  of  the  world 
occur  in  vast  areas,  covering  hundreds  of  thousands 
of  square  miles.  There  are  some  less  extensive 


SUMMARY  OF  HISTORICAL  GEOLOGY    119 

schists  that  were  formed  in  post-Archean  times, 
but  no  schists  constituting  wide  regions  have  yet 
been  proved  to  have  been  formed  in  a  time  later 
than  the  Archean. 

Above  the  crystalline  rocks  of  the  Eozoic  Group 
is  a  thick  series  of  sedimentary  rocks,  mainly 
conglomerates,  sandstones,  and  shales,  which  are 
often  no  more  altered  than  many  sandstones  of 
much  later  dates.1  They  are,  however,  practically 
barren  of  fossils,  and  are,  therefore,  often  included 
in  the  Archean.  Some  of  the  Scottish  sandstones 
of  this  Group  were  once  regarded  as  belonging 
to  the  much  younger  Old  Red  Sandstone  (p.  121). 
These  rocks  are  so  little  altered  that  if  animals 
with  hard  shells  had  lived  during  their  deposi- 
tion, there  is  no  reason  that  fossils  should  not 
have  been  preserved.  Hitherto,  however,  these 
rocks  have  proved  almost  unfossiliferous.  The 
world  was  not  uninhabited  during  their  deposition, 
for  they  yield  occasional  fossils,  such  as  Beltina,  a 
crustacean  found  in  Montana;  and  the  Torridon 
Sandstones  of  Scotland  include  phosphatic  grains 
which  retain  traces  of  organic  structure. 

The  Archean  limestones  may  have  been 
formed  by  some  living  agency,  and  the  Archean 
graphite  and  hydrocarbons  may  be  the  last 
stage  in  the  alteration  of  some  seaweeds.  We 
are,  however,  still  without  any  but  the  scantiest 
knowledge  of  life  before  the  Palaeozoic  Era.  It 
is  probable  that  most  of  the  creatures  living 
before  that  date  had  no  shells  or  hard  skeletons, 
and  thus  left  no  direct  traces  of  their  existence, 

1  These  rocks  in  America  are  included  in  the  Algonkian 
System,  and  that  term  has  been  used  for  the  equiva- 
lent rocks  in  some  parts  of  Europe.  They  are  repre- 
sented in  the  British  Isles  by  the  Torridon  Sandstone  of 
north-western  Scotland.  i-.N 


120  GEOLOGY 

The  Archean  division  is  sometimes  called  the 
Azoic — that  is,  without  life;  but  it  is  rather  the 
period  in  which  the  animals  were  without  shells. 

The  Palaeozoic  Group  includes  six  Systems,  all 
,of  which  contain  abundant  fossil  remains.  The 
oldest  of  the  Systems  is  the  Cambrian,  which 
was  so  named  from  its  development  in  North 
Wales.  Its  rocks  have  yielded  abundant  fossils, 
including  representatives  of  all  the  chief  groups 
of  the  animal  kingdom  except  the  bone-bearing 
animals  (the  vertebrates);  and  the  fact  that  so 
many  distinct  groups  of  animals  were  then  in 
existence  shows  that  life  must  have  existed  on 
the  earth  for  a  very  long  period  previously. 

The  Cambrian  rocks  mostly  consist  of  coarse 
sediments,  laid  down  as  shore  deposits  or  in  shallow 
.seas.  There  were  numerous  volcanic  eruptions 
during  this  period,  and  the  rocks  have  been  con- 
siderably altered  by  earth  movements,  so  that  the 
clays  that  were  then  formed  have  been  pressed  into 
slates.  Limestones  were  not  abundant,  but  they 
occur  in  some  localities  as  in  the  north-west  of 
Scotland.  The  most  characteristic  animals  of 
this  period  were  the  trilobites,  upon  the  succes- 
sion of  which  the  system  is  subdivided,  and  the 
various  subdivisions  correlated  in  different  parts 
of  the  world. 

The  Ordovician  System  was  named  after  the 
tribe  of  the  Ordovices,  who  lived  in  Shropshire 
and  along  the  Welsh  border.  The  Ordovician 
Period  was  marked  by  intense  volcanic  activity 
beginning  with  great  eruptions  in  the  Lake  District, 
while  the  sea  covered  southern  Scotland  and 
Wales.  By  the  close  of  the  period  the  volcanic 
centre  had  moved  from  the  Lake  District  to  North 
Wales,  and  the  eruptions  there  discharged  the 


SUMMARY  OF  HISTORICAL  GEOLOGY    121 

mass  of  volcanic  rocks  that  form  the  mountain 
of  Snowdon.  The  characteristic  fossils  of  the 
Ordovician  rocks  are  graptolites. 

The  Silurian  System  was  also  named  after  a 
tribe  that  lived  in  the  border  country  between 
England  and  Wales.  The  Silurian  Period  was 
characterised  by  the  comparatively  quiet  deposi- 
tion of  sediments  and  limestones  in  a  widespread 
sea.  Volcanic  activity  was  practically  dormant. 
The  typical  rocks  of  the  Period  are  shales  and 
limestones;  and  many  of  the  English  Silurian 
limestones  are  so  rich  in  corals  that  they  must 
have  been  formed  as  coral  reefs  in  a  warm  sea. 
The  life  of  this  Period  marks  a  great  advance 
upon  that  of  the  two  earlier  Periods;  and  it  in- 
cludes the  oldest  known  insects  and  fish,  which 
are  the  first  representatives  of  the  back-boned 
animals. 

The  Devonian.  —  The  Silurian  System  was 
brought  to  a  close  by  its  shales  and  limestones 
gradually  giving  place  to  coarse  sediments  that 
must  have  been  formed  either  along  the  shore, 
or  on  land,  or  in  fresh  water;  and  this  change 
was  due  to  an  emergence  of  the  land  from  the  sea. 
A  continent  was  thus  formed  in  the  Devonian 
Period,  and  it  occupied  the  northern  part  of  the 
North  Atlantic  and  included  all  Europe  north  of 
a  line  across  southern  Ireland,  along  the  Bristol 
Channel,  and  the  valley  of  the  Thames,  and 
thence  across  Belgium  and  Germany  to  the 
.Gulf  of  Finland  in  Russia.  This  continent  ex- 
tended so  far  northward  as  to  include  Spitsbergen. 
The  chief  deposits  formed  upon  this  land  were  the 
thick  series  of  sandstones  known  as  the  Old  Red 
Sandstone.  South  of  this  land  lay  a  sea  wherein 
was  deposited  a  series  of  marine  rocks,  which  were 


122  GEOLOGY 

first  recognised  in  Devonshire,  and  so  the  System 
has  been  named  the  Devonian.  The  Devonian 
marine  rocks  were  formed  at  the  same  time  as 
the  Old  Red  Sandstone.  The  earth  movements 
which  upraised  the  Devonian  land  were  continued 
through  the  Period,  and  were  accompanied  by 
volcanic  eruptions  that  piled  up  huge  volcanic 
domes  in  Scotland ;  and  there  were  many  volcanic 
islands,  the  shores  of  which  were  fringed  with  coral 
reefs,  in  the  sea  covering  southern  Devonshire. 
The  sea  and  fresh  waters  then  swarmed  with 
large  fish,  the  most  characteristic  of  which  were 
protected  externally  by  an  armour  of  plates 
of  bone.  As  the  fish  were  the  dominant  forms  of 
life,  the  Devonian  Period  is  often  called  "  the  Age 
of  Fish." 

Carboniferous. — The  Devonian  Period  was 
followed  by  the  Carboniferous,  and  the  change 
was  marked  by  a  submergence  of  England,  Wales, 
and  Ireland,  by  a  great  extension  of  the  sea.  A 
thick  limestone — the  Carboniferous  Limestone — 
crowded  with  corals,  sea-lilies,  and  other  fossils 
that  indicate  a  clear,  open  sea,  was  deposited  on 
its  bed.  The  sea  sometimes  extended  into  southern 
Scotland,  which  for  most  of  the  time  stood  above 
sea  level,  with  occasional  submergences  beneath  a 
shallow  sea.  Thus,  while  the  rocks  of  the  lower 
part  of  the  Carboniferous  System  in  the  south 
of  England  include  a  sheet  of  limestone  some 
thousands  of  feet  thick,  they  consist  in  Scotland 
•of  volcanic  rocks,  thin  beds  of  limestone,  sand- 
stones, and  clays.  There  are  also,  in  the  lower 
Carboniferous  rocks  of  Scotland,  seams  of  coal, 
some  of  which  may  have  been  formed  as  a  forest 
growth  on  land,  as  the  sites  of  some  ancient 
forests  are  found  in  these  rocks, 


SUMMARY  OF  HISTORICAL  GEOLOGY    123 

The 'great  depression  which  led  to  the  forma- 
tion of  the  Carboniferous  Limestone  in  England 
was  followed  by  an  uplift  which  converted  the 
whole  of  the  British  Isles  into  land.  Great 
forests  grew  on  this  land,  and  some  of  their 
vegetation  has  been  preserved  as  the  coal  seams, 
which  are  the  most  important  source  of  British 
mineral  wealth.  Some  of  the  coal  may  have  been 
formed  in  swamps  and  lakes,  but  some  of  it  was 
certainly  formed  on  the  sites  of  forests,  as  the  roots 
of  trees  are  still  found  in  the  clay  beneath  the  coal 
seams. 

The  Carboniferous  Period  in  the  British  Isles 
appears  to  have  had  a  somewhat  warmer  and 
moister  climate  than  the  present,  judging  by  the 
great  development  of  corals  in  the  Carboniferous 
limestone  sea,  and  the  luxuriance  of  the  vegetation 
in  the  Coal  Measure  forests.  In  the  southern 
hemisphere,  on  the  other  hand,  the  climate  may 
have  been  colder  than  at  present.  In  the  Carbon- 
iferous, Permian,  and  some  later  periods  a  vast 
continent  extended  from  eastern  Brazil  to  Australia, 
including  southern  Africa,  the  Indian  Ocean,  and 
southern  India.  It  is  called  Gondwanaland,  from 
the  Gondwana  beds  of  India.  Some  of  the  moun- 
tains of  this  continent  were  capped  by  perpetual 
snow,  and  great  glaciers  flowed  down  from  them 
on  to  the  lowlands.  There  is  no  direct  evidence 
that  the  ice  then  reached  the  sea  in  Africa  or  India, 
but  icebergs  at  this  date  floated  in  the  sea  further 
north  than  Sydney  in  eastern  Australia,  and  in 
West  Australia  glacial  beds  were  laid  down 
inters tratified  with  marine  deposits.  The  climate 
of  Gondwanaland  was,  therefore  apparently 
colder  than  that  of  the  existing  fragments  of  that 
continent,  though  there  is  no  evidence  of  cold 


124  GEOLOGY 

conditions  in  Europe  at  the  time  when  these 
glaciers  existed  in  southern  Africa,  India,  and 
Australia, 

The  Permian. — The  Carboniferous  Period  was 
followed  by  a  time  of  great  earth  movement 
and  volcanic  activity,  during  which  a  range  of 
mountains  (the  Armorican-Variscan  chain)  was 
raised  across  central  Europe,  extending  from  the 
south  of  Ireland  and  Brittany  eastward  into 
Germany. 

In  addition  to  the  folds  that  formed  the  eastern 
and  western  chain,  a  series  of  movements,  along 
faults  that  ran  north  and  south  uplifted  and 
tilted  great  blocks  of  the  crust,  and  thus  formed 
the  Pennine  Range.  These  combined  earth 
movements  enclosed  an  inland  sea  that  extended 
from  Germany  into  northern  England.  In  this 
sea  were  laid  down  thick  beds  of  dolomitic  lime- 
stones, red  shales,  and  red  sandstones.  The 
animals  that  lived  in  the  sea  of  this  period  were 
mainly  stunted  survivors  from  the  rich  Carboni- 
ferous fauna;  but  the  steady  progress  in  the 
evolution  of  life  with  the  advance  of  time  was 
marked  by  the  varied  forms  of  reptiles  and 
amphibians  that  lived  both  on  land  and  in  water 
in  the  Permian  Period. 

THE  MESOZOIC 

The  Permian  was  the  last  System  belonging  to 
the  Palaeozoic,  and  its  successor,  the  Trias,  was  the 
first  of  the  three  Systems  of  the  Mesozoic — the 
middle  Era  in  the  history  of  life  on  the  earth. 

The  Trias. — The  Permian  movements  had  con- 
verted most  of  northern  Europe  into  part  of 
a  great  continent.  The  most  characteristic  de- 


SUMMARY  OF  HISTORICAL  GEOLOGY    125 

posits  laid  down  on  this  land  were  sheets  of 
red  shales  and  sandstones,  grouped  together  as 
the  New  Red  Sandstone.  Arms  of  the  sea  were 
cut  off  in  this  land ;  their  waters  gradually  evapo- 
rated and  left  the  rich  deposits  of  gypsum  and 
rock  salt  which  are  now  mined  in  Cheshire  and 
Worcestershire,  and  supply  most  of  the  salt  used 
in  the  British  Isles.  The  formation  of  these  beds 
of  salt  shows  that  the  climate  must  have  been 
very  dry,  as  the  evaporation  would  not  have 
taken  place  in  a  rainy  country.  The  south-west 
winds  were  then  prevalent  in  the  British  Isles 
as  at  present.  These  winds  now  bring  us  most 
of  our  rain,  so  they  must  then  have  been  dried 
before  reaching  the  British  area;  they  had  pro- 
bably lost  their  moisture  by  passing  over  land 
that  extended  far  out  into  the  Atlantic,  and 
perhaps  by  crossing  over  the  high  mountains 
formed  by  the  Permian  movements. 

The  British  Isles  in  the  time  of  the  New  Red 
Sandstone  were  a  desert,  and,  as-shown  in  Leicester- 
shire, its  surface  was  carved  by  blown  sand,  showing 
forms  due  to  wind  erosion  similar  to  those  of  exist- 
ing deserts.  In  Germany  somewhat  the  same  con- 
ditions prevailed  through  most  of  the  Triassic  time, 
but  they  were  interrupted  by  a  temporary  occupa- 
tion of  the  country  by  sea.  Further  to  the  south, 
in  eastern  Switzerland  and  western  Austria,  all 
the  Triassic  rocks  were  formed  in  a  sea  that  was 
the  forerunner  of  the  Mediterranean. 

Fossils  are  usually  scarce  in  the  British  New 
Red  Sandstone,  but  there  are  footprints  of 
great  land  animals,  bones  of  crocodiles,  and 
occasional  shells  of  Crustacea,  such  as  water  fleas, 
similar  to  those  that  live  in  pools  or  salt  lakes  in 
Central  Australia.  All  the  characteristic  forms  of 


126  GEOLOGY 

Palaeozoic  life  had  become  extinct,  more  modern 
types  appearing  to  take  their  place. 

The  Jurassic. — The  Trias  was  followed  by  the 
Jurassic;  in  contrast  to  the  Triassic,  this  was 
essentially  a  marine  period.  The  sea  submerged 
the  land  in  a  series  of  successive  advances.  The 
Jurassic  System  takes  its  name  from  the  Jura 
Mountains,  where  these  rocks  are  well  developed. 
The  English  Jurassic  rocks  are  marine  lime- 
stones, sands,  and  clays.  The  limestones  are  often 
oolitic  in  structure,  (see  p.  52),  and  furnish  many  of 
the  most  valuable  of  English  building-stones.  The 
first  birds  make  their  appearance  in  this  period, 
and  there  are  occasional  remains  of  mammals; 
but  this  was  especially  "  the  Age  of  Reptiles,"  of 
which  many  then  lived  on  land  or  in  the  sea,  while 
some  flew  in  the  air.  Some  of  the  largest  of  these 
Jurassic  reptiles,  lived  in  swamps,  and  were  much 
longer  than  any  existing  land  animals.  The 
characteristic  animals  of  the  Jurassic  seas  were  the 
ammonites  and  belemnites,  two  extinct  groups, 
related  respectively  to  the  nautilus  and  cuttlefish. 
Another  characteristic  shell  is  that  of  the  Trigonia, 
which  became  extinct  in  Europe  at  the  close  of  the 
Mesozoic,  but  is  still  living  in  the  seas  of  Australia. 

The  Cretaceous. — The  Cretaceous  System  de- 
rives its  name  from  the  Latin  word  creta,  chalk, 
its  most  characteristic  rock.  The  Cretaceous 
System  is  divided  into  two  well-marked  divisions. 
It  began  in  many  areas  with  land  conditions  due 
to  the  uplift  which  brought  the  Jurassic  Period 
to  a  close.  The  best  known  of  these  continental 
deposits  in  the  British  Isles  occur  in  the  Weald 
of  Kent  and  Sussex.  They  were  probably  laid 
down  on  the  shores  of  a  great  estuary,  and  the 
land  was  clad  with  forests  of  Cycads — trees  allied 


SUMMARY  OF  HISTORICAL  GEOLOGY    127 

to  the  ferns — and  was  the  home  of  gigantic 
reptiles  with  a  kangaroo-like  gait,  such  as  the 
iguanodon.  The  rocks  of  the  upper  part  of  the 
Cretaceous  System  were  marine  in  origin.  The 
Wealden  land  gradually  sank  below  the  sea,  which 
was  deepened  and  widened  till  in  the  time  of  the 
chalk  it  extended  at  least  from  Ireland,  across 
Europe  to  the  Crimea,  and  in  it  was  laid  down 
a  limestone  of  exceptional  purity.  It  contains 
so  little  sedimentary  material  that  it  must  have 
been  formed  far  from  land.  Some  boulders  found 
in  the  chalk  near  London  were  probably  dropped 
there  by  icebergs,  which  had  drifted  southward  and 
there  melted  away. 

The  Jurassic  and  Cretaceous  Periods  in  the 
British  Isles  were  free  from  volcanic  eruptions ; 
but  the  Cretaceous  was  brought  to  an  end  by  great 
earth  movements  which  raised  the  floor  of  the 
chalk  sea  into  land,  and  these  disturbances,  in 
many  parts  of  the  world,  culminated  in  prolonged 
and  widespread  volcanic  activity. 

THE  CAINOZOIC 

The  interval  between  the  top  of  the  chalk  and 
the  deposition  of  the  earliest  beds  of  the  Cainozoic 
must  have  been  very  prolonged :  for  such  influential 
geographical  changes  happened  in  it,  that  the  rich 
Mesozoic  fauna  found  in  the  upper  beds  of  the 
Cretaceous  had  all  become  extinct  before  the  de- 
position of  the  lowest  beds  of  the  Cainozoic.  New 
animals  and  plants  made  their  appearance,  and 
they  were  all  of  more  modern  types,  marking  the 
beginning  of  the  Era  of  recent  life. 
.  The  Eocene. — The  Cainozoic  is  divided  into  five 
Systems,  of  which  the  lowest  is  known  as  the 


128  GEOLOGY 

Eocene  or  "  the  dawn  of  recent  life."  Its  beds 
were  formed  partly  in  shallow  seas  or  along  coasts, 
and  partly  in  lakes  and  on  land.  Marine  deposits 
of  this  Period  occur  in  the  London  and  Hampshire 
basin,  and  include  the  London  Clay,  which  forms 
the  foundation  of  London.  Further  west,  up  the 
Thames  Valley  near  Reading,  and  in  Devonshire, 
in  Ireland,  and  also  in  Scotland,  the  Eocene 
deposits  were  formed  on  land.  A  continent  must 
have  extended  from  western  Scotland  across  the 
Atlantic  towards  Iceland  and  Greenland,  and  this 
great  land  began  to  be  broken  up  by  the  sub- 
sidences that  later  on  formed  the  basin  of  the 
North  Atlantic.  Great  volcanoes  burst  into 
eruption  around  the  sinking  area,  ejecting  the 
first  of  the  sheets  and  piles  of  lava  that  form 
some  of  the  best-known  features  in  the  scenery 
of  the  Scottish  Isles. 

The  Oligocene. — The  Oligocene  Period,  which 
followed  the  Eocene,  was  mainly  a  continental 
formation,  only  one  marine  deposit  of  this  age 
occurring  among  the  fresh-water  and  land  deposits 
then  laid  down  in  the  British  Isles.  A  great  sea, 
the  forerunner  of  the  existing  Mediterranean, 
covered  much  of  southern  Europe,  extending  east- 
ward into  Asia  and  westward  to  the  West  Indies. 
Northern  and  central  France  and  Germany  were 
then  land,  and  pines  in  the  Oligocene  forests 
of  Germany  exuded  a  resin  that  has  been  since 
converted  into  amber. 

The  Miocene. — The  Miocene — the  Period  of  less 
recent  life — in  central  and  southern  Europe  has 
numerous  marine  rocks,  but  northern  Europe  was 
then  occupied  by  land.  This  Period  is  most  re- 
markable as  a  time  of  great  mountain  formation. 
Among  other  mountains  then  raised  are  the  Alps 


SUMMARY  OF  HISTORICAL  GEOLOGY    129 

and  Alpine  system  of  Europe,  the  Atlas  in  northern 
Africa,  and  the  mountains  of  the  Himalayan  system 
in  Asia.  The  climate  of  this  period  appears  to 
have  been  somewhat  warmer  than  at  present, 
judging  by  the  vegetation  that  then  lived  as  far 
north  as  Greenland  and  Spitsbergen. 

The  Pliocene. — The  Pliocene — the  Period  of  more 
recent  life — was  characterised  by  a  gradual  in- 
crease in  the  coldness  of  the  climate  of  north- 
western Europe.  The  chief  deposits  in  this 
System  in  the  British  Isles  are  the  shell  beds 
known  as  the  Crags  of  Suffolk  and  Norfolk.  The 
lower  beds  of  the  series  contain  fossil  shells  belong- 
ing to  genera  not  now  living  in  the  British  seas, 
but  which  survive  in  the  Mediterranean;  these 
southern  shells  disappear  from  the  later  Pliocene 
beds,  and  their  place  is  taken  by  Arctic  shells, 
showing  that  the  British  seas  were  becoming  colder. 

Pleistocene. — The  climate  became  still  more 
severe  in  the  early  part  of  the  succeeding  Period 
—the  Pleistocene — when  the  mountains  of  the 
northern  and  western  parts  of  the  British  Isles 
were  covered  by  perpetual  snow.  Glaciers  flowed 
from  the  mountain  snow-fields  into  the  valleys 
and  on  to  the  plains;  as  the  ice  melted,  these 
plains  were  covered  with  widespread  sheets  of 
glacial  beds,  and  with  sheets  of  clay,  that  had  been 
deposited  in  lakes  formed  by  ice-dammed  rivers. 
The  cold  climate  is  not  only  proved  by  the  nature 
of  the  deposits,  with  their  ice-scratched  stones, 
and  by  the  ice-worn  surfaces  of  the  rocks,  but  also 
by  the  animals  and  plants  whose  remains  are 
found  associated  with  these  deposits.  Remains 
of  'the  musk  ox,  which  now  lives  in  northern 
Greenland,  in  the  northernmost  parts  of  North 
America  and  in  its  adjacent  islands;  of  the 

i 


130  GEOLOGY 

reindeer,  which  now  inhabit  only  the  colder 
regions  of  America  and  Europe;  and  of  the  hairy 
mammoth  which,  akin  to  the  elephant  but 
covered  with  thick  hair,  ceased  its  wanderings  in 
northern  Siberia — remains  of  all  these  have  been 
found  in  the  British  glacial  and  post-glacial 
deposits.  The  plants  found  in  the  peat  beds 
associated  with  the  glacial  beds  include  such  far 
northern  plants  as  the  Arctic  willow,  and  some 
that  now  live  only  at  sea  level  in  the  Arctic  regions 
or  high  up  on  the  mountains  of  the  British  Isles 
and  on  the  Alps.  The  occurrence  of  these  plants 
in  beds  formed  in  the  lowlands  shows  that  the 
British  climate  was  then  much  colder  than  at 
present. 

The  glacial  conditions  existed  longer  in  Scotland 
than  in  England,  which  enjoyed  a  mild  climate, 
while  the  Scottish  mountains  were  still  snow- 
covered,  and  glaciers  flowed  down  the  western 
valleys  to  the  sea.  The  land  of  the  British  Isles 
was  then  part  of  the  Continent;  many  English 
rivers,  such  as  the  Thames  and  those  from  the 
Wash  and  Humber,  were  tributaries  to  the  Rhine, 
which  was  prolonged  northward  across  the  plain 
that  is  now  the  bed  of  the  North  Sea. 

Such  were  the  geographical  conditions  when 
man  first  entered  the  British  area.  It  had  a 
colder  climate  than  at  present,  for  glaciers  still 
existed  on  the  Scottish  mountains.  He  wandered 
into  the  country  overland  from  the  Continent. 
That  he  was  a  contemporary  of  the  mammoth  and 
reindeer  is  proved  by  carvings  found  in  some  of 
the  caves  occupied  by  these  early  men.  Reindeer 
horns  with  carved  imitations  of  the  reindeer,  and 
sketches  of  the  mammoth  and  reindeer  engraved 
on  fragments  of  mammoth  tusk  (Fig.  40),  proved 


SUMMARY  OF  HISTORICAL  GEOLOGY    131 


that   the  men  who  made  them  were  acquainted 
with  those  animals. 

The  earliest  inhabitants  of  what  are  now  the 
British  Isles  had  no  domestic  animals,  and  they 
did  not  know  the  use  of  metals.  They  lived  only 
by  the  chase  and  fishing,  and  had  implements  of 
wood  and  bone  which  they  shaped  with  tools 
made  of  chipped  stone.  Owing  to  the  primitive 


Fig.  40. 

Sketch  of  a  Reindeer,  engraved  on  a  piece  of  Mammoth  tusk, 
by  Palaeolithic  man.  From  the  Kessler  Loch,  near  Schaffhausen, 
Switzerland. 

character  of  their  stone  tools,  these  people  are 
described  as  "  Palaeolithic" — that  is,  belonging  to 
the  older  Stone  Age.  They  did  not  live  in  Scotland, 
as  the  climate  was  probably  then  still  too  severe. 

These  people  were  succeeded  by  the  Neolithic 
men,  or  those  of  the  later  Stone  Age,  who  had 
discovered  how  to  make  much  better  stone  tools 
by  grinding  their  edges  smooth  and  sharp.  The 
climate  had  now  become  warmer  throughout  the 
British  area,  so  that  although  a  few  Scottish 
glaciers  still  reached  sea  level,  Neolithic  man 
inhabited  Scotland  as  well  as  England. 


132  GEOLOGY 

The  Neolithic  people  were  succeeded  by  races 
who  could  work  in  metal,  and  the  next  Age  is 
known  as  the  Bronze  Age,  because  the  metal  tools 
which  survive  from  it  are  mostly  made  of  bronze. 
It  is  probable  that  iron  implements  were  made 
earlier  than  those  of  bronze,  but  iron  rusts  away 
so  readily  that  they  have  not  survived.  The 
people  of  the  Bronze  Age  spread  across  Europe, 
and  some  of  the  most  complete  collections  of  their 
domestic  articles  and  tools  have  been  obtained  from 
the  sites  of  ancient  villages  built  on  piles  in  the 
Swiss  lakes.  One  group  of  these  Bronze  people 
reached  Great  Britain,  which  was  now  separated 
from  the  Continent,  around  the  coasts  of  western 
Europe.  They  appear  to  have  been  sun  wor- 
shippers, and  in  connection  with  their  religious 
rites  they  built  temples  such  as  Stonehenge, 
which  probably  dates  from  about  1800  years  B.C. 

The  successors  to  the  people  of  the  Bronze  Age 
used  iron  in  preference  to  bronze,  as  it  is  a  more 
serviceable  metal  and  more  abundant  than  the 
tin  and  copper  of  which  bronze  is  made ;  and  with 
the  Iron  Age  the  story  of  the  earth,  as  far  as 
concerns  the  British  Isles,  passes  from  the  sphere 
of  Geology  to  that  of  History. 


GLOSSARY 


ACID  (Latin  acidus,  sour).     This  term  is  applied  in  geology  to 

aqueous  rocks  containing  a  large  proportion  of  silica. 
/EOLIAN  (Greek  Aiolos,  god  of  the  winds).      Deposits  formed 

on  land  by  the  action  of  the  wind. 
ALGONKIAN  (named  after  a  tribe  of  North  American  Indians). 

The    geological    System    that    includes    the    sedimentary 

rocks,   younger    than   the   Archean   and   older  than    the 

Palaeozoic. 
AMPHIBOLES   (L.   amphibolum,   ambiguous,   from  the  variable 

nature  of  the  mineral).     A  group  of  minerals  important  as 

constituents  of  many  rocks.     Most  of  the  species  contain 

iron  and  magnesium.     Hornblende  is  the  typical  species. 
ANDESITE  (named  from  the  Andes,  where  it  is  abundant).     A 

lava  intermediate  in  composition  between  those  rich  in 

silica  and  those  poor  in  silica. 
ANTICLINE  (Gr.  anti,  on  opposite  sides,  and  klino,  I  bend).     An 

archlike  fold  of  stratified  rocks. 
APATITE    (Gr.    apate,    deceit,   and   lithos,    stone).     A  mineral 

species  composed  mainly  of  phosphate  of  lime. 
ARCHEAN  (Gr.  archaios,  ancient).     The  earliest  subdivision  of 

the  Eozoic  rocks. 
ARENACEOUS  (L.  arena,  a  sand  grain).     Rocks  composed  of  sand 

grains. 
ARGILLACEOUS  (from  Gr.  argillos,  clay;   L.  argilla}.     Rocks  and 

beds  composed  of  clay. 
ARTHROPODS  (Gr.  arthron,  a  joint,  and  podes,  feet).     Animals 

with  jointed  limbs. 
AUGITE   (Gr.   auge,  lustre).     The  commonest  mineral  species 

belonging  to  the  group  of  the  pyroxenes. 
BARYSPHERE  (Gr.  barus,  heavy,  and  sphaira,  a  sphere).     The 

central  mass  of  the  earth,  so  named  on  account  of  its  weight. 
BASALT  (word  of  African  origin).     Basic  lava  or  dyke  rock. 
BASE  (Gr.  basis,  foundation).     A  material  that  combines  with 

an  acid  without  losing  anything. 
BASIC.     This  term  is  applied  in  geology  to  igneous  rocks  rich  in 

bases  and  containing  a  comparatively  small  proportion  of 

silica. 
BRACHIOPODS  (Gr.  brachion,  arm,  and  podes,  feet).     A  class  of 

marine  animals  with  a  shell  composed  of  two  valves;  the 

body  has  a  number  of  arm-like  processes,  whence  the  name. 
BRECCIA  (Italian,  a  breach  in  a  wall).     Rock  composed  of  coarse 

angular  fragments. 

133 


134  GEOLOGY 

BRYOZOA  (Gr.  bruona,  mossy,  and  zoon,  a  living  being).      A 

class  of  compound  animals  usually  growing  in  tufts  or  thin 

sheets. 
CAINOZOIC  (Gr.  kainos,  recent,  and  zoe,  life).     The  Period  of 

recent  life — the  name  of  the  last  of  the  four  geological  Eras. 
CALCAREOUS  (L.  adj.  calcarius,  from  calx,  lime).     Composed  of 

carbonate  of  lime. 
CLASTIC   (Gr.   klastos,  broken  to  pieces).     Sedimentary  rocks 

composed  of  fragments. 
CLEAVAGE  (Gr.  klao,  to  break  in  pieces).     The  property  of  which 

some  minerals  or  rocks  break  in  with  smooth  flat  surfaces. 
CCELENTERATA  (Gr.  koilos,  hollow,  and  entera,  entrails,  guts). 

The  group  of  multicellular  animals  in  which  there  is  no 

digestive  cavity  separated  from  a  distinct  body  cavity. 
C(ELOMATA  (Gr.  koiloma,  a  hollow  cavity).     The  group  of  imilti- 

cellular  animals  in  which  there  is  a  digestive  tube  distinct 

from  the  body  cavity. 

CONGLOMERATE  (L.  conglomero,  I  roll  together).     A  rock  com- 
posed of  rounded  pebbles. 
CRINOID  (Gr.  knnon,  a  lily,  and  ei'dos,  likeness).     A  sea  lily. 

The  Crinoidea  are  a  class  of  Echinoderms. 
CRYSTALLINE  (Gr.  krustallos,  ice).     Composed  of  crystals. 
DIATOMS   (Gr.   dia,  through,   and  tome,   a  cut).     A  group   of 

minute  aquatic  plants  with  siliceous  shells,  each  of  two 

valves. 
DIORITE  (Gr.  dioros,  a  well-marked  distinction).     The  typical 

plutonic  rock  of  the  sub-basic  group. 
DOLERITE  (Gr.  doleros,  deceptive,  and  lithos,  a  stone).      The 

basic  igneous  rock  less  coarse  grained  than  gabbro. 
DOLOMITE    (named   after   a   French   mineralogist,   Dolomieu). 

The  mineral  species  or  rock  composed  of  approximately 

equal   amounts   of  carbonate   of  lime   and   carbonate   of 

magnesia. 
ECHINODERMS  (Gr.  echinos,  the  sea  urchin,  and  derma,  a  skin). 

A  group  of  Ccelomate  animals  with  spine-bearing  skin  or 

shell. 
EOCENE  (Gr.  eos,  the  dawn).     The  dawn  of  recent  life,  the  first 

System  of  the  Cainozoic. 
Eozoic  (Gr.  eos,  the  dawn,  and  zoon,  a  living  being).     The  Era 

of  the  dawn  of  life.     The  name  of  the  oldest  of  the  four 

geological  Eras. 
FELSPARs"(fr°m  German  fete,  rock,  andspath,  a  spar  or  mineral). 

A  group  of  rock-forming  minerals. 
FELSPATHOID  (from  felspar,  and  Gr.  eidos,  likeness).     A  group 

of  minerals  that  may  replace  the  felspars  in  igneous  rocks. 
FEMIC  (Fe,  the  symbol  for  iron  (L.  ferrum),  and  M  stands  for 

magnesium).     A   term   applied   to   substances  containing 

much  iron  and  magnesium. 
FOLIATION  (L.  folium,  a  leaf).     The  arrangement  of  the  minerals 

in  a  crystalline  rock  in  parallel  layers. 
FORAMINIFERA   (L.  foramen,   an   opening,   and  fero,   I   bear). 


GLOSSARY  135 

Microscopic  unicellular  animals  that  live  in  the  sea;  their 
shells  are  important  constituents  of  many  limestones. 

GABBRO  (Italian).     The  typical  basic  plutonic  rock. 

GRANITE  (It.  granito).     The  typical  acid  plutonic  rock. 

GRAPTOLITE  (Gr.  graptos,  marked  with  letters,  and  lithos,  stone). 
An  animal  belonging  to  an  extinct  class,  Hydrozoa. 

HOLOCRYSTALLINE  (Gr.  holos,  whole).  A  term  applied  to  rocks 
wholly  composed  of  crystalline  constituents. 

HORNBLENDE  (German  horn,  metal,  and  blenden,  to  deceive, 
because  containing  no  metal,  although  of  a  metallic  lustre). 
The  chief  mineral  species  of  the  group  of  the  Amphiboles. 

HYDROZOA  (Gr.  hydra,  the  water-snake,  from  hudor,  water,  and 
zoon,  a  living  being).  A  class  of  Ccelenterate  animals,  in- 
cluding the  hydra,  the  sea-firs,  and  the  extinct  graptolites. 

ISOCLINE  (Gr.  isos,  equal,  and  klino,  I  bend).  A  fold  in  which 
both  sides  are  inclined  in  the  same  direction. 

KERATOPHYR  (Gr.  kerata,  horns;  phyr  is  adopted  from  the  name 
"  porphyry,"  which  meant  purple,  and  was  given  by  the 
Greeks  to  a  purple  igneous  rock).  An  acid  igneous  rock 
rich  in  soda  or  other  alkali. 

LEUCITE  (Gr.  leukos,  white).  A  white-coloured  mineral  espe- 
cially abundant  in  some  Italian  lavas. 

LIPARITE  (named  from  its  abundance  in  the  Lipari  Islands).  A 
lava  rich  in  silica. 

LITHOSPHERE  (Gr.  lithos,  stone,  and  sphaira,  a  sphere).  The 
stony  crust  of  the  earth. 

MAGNETITE.     An  oxide  of  iron  with  strong  magnetic  properties. 

MEROCRYSTALLINE  (Gr.  meros,  a  part).  A  term  applied  to  rocks 
partly  composed  of  crystalline  constituents. 

MESOZOIC  (Gr.  mesos,  middle,  and  zoe,  life).  The  middle  Era  of 
life;  the  third  of  the  four  Eras  into  which  geological  time  is 
divided. 

METAMORPHISM  (Gr.  meta,  after,  as  in  physics  and  metaphysics; 
and  morphe,  form).  A  change  in  a  rock  which  alters  the 
arrangement  of  its  materials,  but.  not  its  composition. 

METASOMATISM  (Gr.  meta,  after,  and  soma,  a  body).  A  change  in 
a  rock  which  alters  its  composition. 

METAZOA  (Gr.  meta,  after,  and  zoe,  life).  The  sub-kingdom  of 
animals  in  which  each  animal  is  composed  of  many  cells. 

MICA  (L.  mico,  I  glitter).  A  group  of  mineral  species  charac- 
terised by  breaking  into  thin  glistening  flakes. 

MINERALS. — The  inorganic  constituents  of  the  earth's  crust. 
Mineral  species  are  minerals  that  cannot  be  broken  up  into 
other  kinds  of  minerals  by  mechanical  processes. 

MIOCENE  (Gr.  meion,  less,  and  kainos,  recent).  The  middle 
Period  of  recent  life;  the  middle  Period  of  the  Cainozoic 
Era. 

MOLLUSCA  (L.,  meaning  a  soft  nut  with  a  thin  shell,  from  mollis, 
soft).  The  group  of  animals  including  the  "  shell-fish/' 

MONOCLINE  (Gr.  monos,  single,  and  klino,  I  bend).  A  fold  with 
only  one  side. 


136  GEOLOGY 

NEBULA  (L.,  meaning  a  vapour,  cloud).     A  star  composed  of 

either  a  cloud  of  gas  or  swarm  of  meteoritic  masses. 
NEPHELINE  (Gr.  nephile,  a  cloud).     A  mineral  species  found  in 

some  igneous  rocks  rich  in  soda. 
OLIGOCENE  (Gr.  oligos,  little,  and  kainos,  recent).     The  second 

in  time  of  the  five  Systems  of  the  Cainozoic  Group. 
ORTHOCERAS    (Gr.    orthos,    straight,    and    keras,    a    horn).     A 

chambered  shell  like  a  straight,  uncoiled  nautilus. 
PALEOZOIC  (Gr.  palaios,  ancient,  and  zoe,  life).     The  Era  of 

ancient  life.     The  second  of  the  four  geological  Groups  and 

Eras. 
PHONOLITE  (Gr.  phonea,  sound,  and  lithos,  stone).     A  lava  rich 

in  soda:   it  is  used  for  rock  harmonicons,  hence  its  name. 
PLEISTOCENE  (Gr.  pleistos,  most,  and  kainos,  recent).     The  last 

of  the  five  Systems  of  the  Cainozoic  Group. 
PLIOCENE  (Gr.  pleion,  more,  and  kainos,  recent).     The  fourth  in 

time  of  the  five  Systems  of  the  Cainozoic  Group. 
PLUTONIC  (named  after  Pluto,  the  God  of  the  infernal  regions). 

The   igneous  rocks  that  have  solidified  deep    below   the 

surface  of  the  earth. 
PROTOZOA  (Gr.  protos,  first,  and  zoe,  life).     The  lowest  group  of 

the  animal  kingdom,  including  animals  composed  of  only 

one  cell,  or  of  a  few  exactly  similar  cells. 
PYROXENES  (Gr.  pur,  fire,  and  zenos,  a  stranger).     A  group  of 

mineral  species  found  mostly  in  basic  igneous  rocks. 
RADIOLARIA   (L.   radiolus,   a  little  rod).     A   group   of    simple 

animals  belonging   to  the   Protozoa,   and  usually  having 

siliceous  shells. 
REGIME  (French).     The  condition  reached  by  a  river  when  it 

neither  wears  away  its  bed  nor  deposits  material  on  it. 
RHYOLITE   (Gr.  rhuo,   I  flow,  and  lithos,  stone).     An   altered 

liparite. 
SCREE  (from  Anglo-Saxon  scrithan,  to  go  or  creep).     The  same 

as  Talus. 

STRATUM  (L.,  a  layer).     A  layer  or  bed  of  rock. 
SYENITE   (from  Syene,   a  locality  on  the   Nile).     The  typical 

plutonic  rock  of  the  Sub- acid  Group. 
SYNCLINE  (Gr.  syn,  together,  and  klino,  I  bend).     A  trough-like 

fold  of  stratified  rock. 
TALUS  (L.  talus,  the  heel).     The  collection  of  broken  rocks  at  the 

foot  of  a  cliff  or  steep  slope. 
TEPHRITE  (Gr.  tephra,  ashes,  and  lithos,  stone).     A  basic  lava 

rich  in  soda. 
ZIRCON  (Arabic  zarkun,  vermilion).     A  mineral  species  found  as 

small  crystals  in  many  igneous  rocks.     Large  crystals  are 

used  as  gems 


INDEX 


^Eolian  deposits,  21,  40,  133 
Agglomerate,  100 
Alabaster,  49 

Algae  as  rock  formers,  52,  53 
Algonkian  System,  119,  133 
Alps,  date  of' formation  of,  128 
Amber,  a  fossil  resin,  128 
Ammonite,  114,  116,  126 
Amphibia,  114 
Amphiboles,  26,  133 
Andesite,  28,  34,  35,  37,  133 
Anthozoa,  in 
Anthracite,  56,  57 
Anticline,  96,  133 
Apatite,  55,  133 
Aqueous  deposits,  21,  40,  41- 

49,  80-87 

Armorican-Variscan  chain,  124 
Arthropods,  112,  133 
Augite,  26,  27,  36,  133 
Azoic,  1 20 

Barysphere,  18,  19,  133 
Basalt,  22,  24,  28,  34,  35,  37, 

93,  133 
Basanite,  34 
Base,  25,  133 
Base  level  of  a  river,  69 
Basic  rocks  and  minerals,  25, 

26,  27,  29 
Bedding,   current,    90;     false, 

90;   planes,  89 

Belemnite,  114,  115,  116,  126 
Beltina,  119 
Boulder  clay,  86 
Brachiopods,  112,  133 
Breccia,  42,  133 
Bronze  Age,  132 
Bryozoa,  52,  112,  134 
Building-stones,  decay  of,  16; 

oolitic,  52 


Cainozoic,  118,  127,  132,  134 

Calcareous  rocks,  44-46,  51, 
52,  134 

Cambrian  System,  120 

Cannel  coal,  57 

Carbonaceous  rocks,  55-58; 
chemical  composition  of,  57 

Carbonates,  chemically  de- 
posited, 44-47 

Carboniferous  Period,  122-124 

Cave,  formation  of  a,  45,  71 

Chalk,  126,  127,  162;  Fig.  6r 
P.  63 

Chert,  53 

Clay,  38,  39,  42-43 

Cleavage,  59,  134 

Clinometer,  89 

Coal,  formation  and  varieties, 
55-57;  age  of,  123 

Coelenterata,  no,  134 

Coelomata,  in,  134 

Contact-aureole,  59,  99 

Contact-metamorphism,  59 

Continental  slope,  deposits  of, 
85 

Coprolites,  55 

Coral  islands,  41 

Corals,  51,  in 

Corrosion,  67-69;   glacial,  75 

Cretaceous,  126 

Crinoids  (sea  lilies),  51,  134 

Crystalline,  23,  134 

Cycads,  126 

Deltas,  81-84 

Denudation,    63;     agents    ofr 

63,  77;   slowness  of,  61 
Derbyshire  marble,  52 
Devonian  Period,  121-122 
Diatoms,  53,  134 
Diorite,  27,  28,  33,  35,  37,  134 


137 


138 


GEOLOGY 


"  Dip,"  89 

Dolerite,  28,  37,  134 

Dolomite,  47,  134 

Dunes,  41,  79 

Dykes,  28,  99;   Fig.  27,  p.  98 

Dynamo-metamorphism,  59 

Earthquakes,  101-103 
Echinoderms,  112,  134 
Eocene  System,  127,  134 
Eozoic  life,  119,  134 
Erosion,  66,  67,  69 
"  Erratics,"  77 
Etna,  Mount,  15 
Evaporation,  rocks  formed  by, 

48-49 

Explosions,  volcanic,  earth 
shaken  by,  103 

False  bedding,  90 

Faults,  97;  ridge,  97;  step,  96; 

trough,  96 
Felsite,  31 
Felspar,  25,  27,  30,  33,  34,  35, 

39,  134 

Felspathoids,  25,  134 

Femic,  26,  37,  134 

Fish,  114 

Flood  plain,  84 

Folds,  95-97 

Foliation,  59,  134 

Foraminifera,  52,  53,  109,  134 

Fossils,  104-116;  evidence  re- 
garding geographical  and 
climatic  conditions,  115 

Gabbro,    27,    28,    33,    34,    35, 

37,  39,  135 
Glaciers,  16,   129;    action  of, 

75 

Glass  in  rocks,  22,  23,  24,  31 
Gneiss,  60 

Gondwanaland,  123;   map,  8 
Granite,   23,   24,   27,   28,   30, 

31,  33,  34,  35,  37,  38,  39,  J35 
Graphite,  57 

Graptolites,  in,  116,  121,  135 
Gravel-pits,  n 
Grit,  23,  42 
Guano,  54 


Holocrystalline,  24,  135 
Hornblende,  27,  35,  36,  137 
Horst,  97 
Hydrosphere,  19 
Hydrozoa,  in,  135 

Ice  action,  74-77 

Iguanodon,  127 

Insects,  oldest  known,  121 

Iron  Age,  132 

Iron  ores,  47 

Ironstone,  42 

Isocline,  96,  135 

Joints,  92-93;    Fig.  26,  p.  98 
Jurassic,  126 

Keratophyr,  34,  135 

Lake  deposits,  84 

Landslips,  71 

Lava,  28,  100 

Leucite,  25,  135 

Lignite,  56 

Limburgite,  28,  35 

Limestone,  Carboniferous,  123 ; 

oolitic,  52,  126 
Limestones,  Archean,  119 
Liparite,  23,  28,  34,  35,  37,  *35 
Lithosphere,  19,  20,  135 
Loess,  79-80 
London  Clay,    128;      Fig.    6, 

P-  63 

Magma,  31 

Magnetite,  26,  36,  135 

Mammals,  115 

Marble,  60 

Marine  deposits,  85-86 

"  Medals  of  Creation,"  fossils 

as,  105 

Merocrystalline,  24,  135 
Mesozoic,  118,  124-127,  135 
Metamorphism,  60,  135 
Metasomatism,  60 ',  135 
Metazoa,  109,  no,  135 
Micas,  25,  27,  30,  35,  38,  135 
Millstone  grit,  42 
Mineral  lodes,  47 
Mineral  species,  24, 135 ;  rock- 
forming,    24-27;     order    of 


INDEX 


139 


crystallisation,   39;    decom- 
position of,  39 

Miocene  Period,  128-129,  135 

Mollusca,  51,  113,  135 

Monocline,  94,  135  • 

Moraines,  73,  76,  86 

Musk  ox,  129 

Nebula   and  nebular   theory, 

12-15,  136 
Neolithic,  131 
Nepheline,  25,  136 
New     Red    Sandstone,     125; 

desert  climate  of,  125 
Nummulites,  no 

Obsidian,  30,  31,  35 

Oil  shale,  57,  58 

Old  Red  Sandstone,  121,  122 

Oligocene  Period,  128,  136 

Olivine,  27,  34,  35,  36 

Oolites,  52,  126 

Ooze,  52,  86 

Ordovician  System,  120 

Ore  deposits,  47-48 

Orthoceras,  113,  136 

Palaeolithic,  131 

PalaBOiitology,  104 

Palaeozoic,  118,  120,  136 

Parallel  Roads  of  Glen  Roy,  87 

Peat,  55 

Pelee,  Mount,  103 

Peneplane,  70 

Pennine  Range,  formation  of, 

124 

Peridotite,  27,  28,  35,  37 
Permian,  124 
Petroleum,  58 
Phonolite,  34,  136 
Phosphatic  deposits,  53-55 
Pitchstone,  35 
Pleistocene,  129-132,  136 
Pliocene,  129,  136 
Pot-holes,  68,  73 
Protozoa,  109,  136 
Pyroxene,  26,  35,  136 

Quartz,  25,  27,  30,  33,  34,  35, 
36,  38,  39,  42 

euartz-felsite,  30,  31 
uartzite,  60 


Radiolaria,  53,  no,  115,  136 

Red  clay,  86 

Regime  of  a  river,  81,  136 

Reptiles,  114;   age  of,  126 

Rhyolite,  30,  31,  136 

Rift  valley,  97 

River  deposits,  80- 8 1 

River  fan,  81 

Roches  moutmnees,  76 

Rock  salt,  125 

Rocks,  acid,  33,  36,  133; 
aeolian,  40;  aqueous,  21, 
40,41-47,48;  archean,  118, 
133;  arenaceous,  41,  133; 
argillaceous,  41,  42,  133; 
arrangement  in  the  field,  87- 
93;  basic,  33,  36,  133;  cal- 
careous, 44,  51;  carbona- 
ceous, 55-58;  clastic,  40, 
134;  crystalline,  21;  de- 
struction and  decay  of,  63; 
efflorescent,  49 ;  eozoic,  117; 
igneous,  21-37,  43;  meta- 
morphic,  58-60;  organi- 
cally formed,  50-60;  pri- 
mary, 22-37,  40,  41,  43; 
plutonic,  22,  24,  27-37,  99, 
136;  secondary,  20,  21,  37- 
40,77-87;  sedimentary,  40; 
siliceous,  44,  52,  53;  strati- 
fied, 21,  37-40,  43;  sub- 
aerial,  21,  40;  volcanic,  22, 
24 

St.  Cuthbert's  beads,  51 

Sand,  41,  42;  erosion,  64 

Schists,  60 

Screes,  79,  136 

Sea  as  denuding  agent,  72 

Sea  lilies  (Crinoids),  51,  112 

Sea  salt,  composition  of,  48; 

precipitation  of,  85 
Sedimentary  deposits,  41-43 
Serpentine,  33 
Silurian  System,  121 
Smith,  William,  his  discovery, 

105 

Soils,  78 

Specific  gravity  of  rocks,  36-37 
Sponges,  no 
Springs,  46,  70 


140 


GEOLOGY 


Stonehenge,  132 

Stratum  (pi.  strata),  20,  136 

Strike,  89 

Subaerial,  21,  78 

Subsoils,  78 

Syenite,  27,  28,  33,  35,  37,  136 

Syncline,  94,  96,  136 

Tachylyte,  35 

Talus,  79,  136 

Tephrite,  34,  136 

Thames,  load  transported  by, 

16 

Thermo-metamorphism,  59 
Tilting,  regional,  103 
Torridon  Sandstone,  119 


Trachyte,  28,  34,  35 
Trias,  124-126 
Trigonia,  126 
Trilobite,  113,  116,  120 
Tripoli  Powder,  53 
Tuffs,  100 

Unconformity,  91 

Vertebrata,  114 
Volcanoes,  15,  99-103 

Waterfall,  effect  of,  67 
Worms,  112;  geological  agency 
of,  78 

Zircon,  126,  136 


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